Monday, May 23, 2011

Defense...The basics

"Only steers and queers come from Texas..."
 Reading a lot on the Huey board and finding that a lot of folks don't seem to understand the basics of defensive football.  More particularly, the questions about what defense and why.  I see on there, entirely too much, "Best Defense for Double Wing" or "Best Defense With no Defensive Linemen", "Best Coverage vs. Air Raid Concepts" etc.  To which I roll my eyes and go, "Here we go again!".  I'm really not trying to talk down to anyone, because I used to be the EXACT same way, until one day a light went off in my head, and that's really what I want to share with you.  For those of you that are still adamant that the 3-3 stack is the best defense in football, or NOBODY can run on my 4-4 robber scheme, please, reserve reading this for when you've opened your minds a little more.  For those that want to learn about the basics of defense, stick with me, I won't lead you astray!

I look like a trustworthy guy...right?

Basics- The Scheme
If you are defensive coordinator (DC) the first thing you had to do, was pick what defense you wanted to run.  To some this is an easy task, many pick up a defense they ran in college, or high school, or run what the former DC they learned under ran (which was my case).  I took over for a guy who stepped down to just position coach after 25 years in the business; he had run the 50 Monster and the 4-3 (I called it the Stoops 4-3 because it came from the University of South Florida, when Leavitt was there and Leavitt and Stoops were cut from the same mold) and had very good success with both.  He had moved to the 4-3 from the 50 after the infiltration of the spread, and because of the proximity of where he coached to the University of South Florida (a die hard 4-3 team back in those days).  Well, I had coached defensive line (DL) and defensive backs (DB's) under him, and was named his successor, so I just ran what I knew, his 4-3.  Anyway, enough with history, the basis here is that everyone has to pick a basic scheme to run.  No matter where you get it, you have to pick a base.  Now I'm not going to sit here and tell you to run the 4-3 because it's the most awesomest, gnarliest, bad-to-the-bone defense that every stepped on to a gridiron.  No sir, that's not me, as I firmly believe that all defenses work.  They have to, or else there wouldn't be so many different ones out there.  The golden rule here is "There are no magic bullets".  You WILL NOT install a defense that will make your 5'6" 165 pound defensive end (DE) play like Jevon Kearse.  I'm sorry, I wish that were the case, but it just doesn't work that way.  So where to start?  Let's look at that.  When starting out, I'd look at these key elements when picking a defense:

  1. What do you know?- This is important, as you must be able to fix it when it's broken, and you must be able to not only teach it to your kids, but in some cases, you must be able to teach it to your assistant coaches.  Going with what you know is a good first step in identifying what defense you would like to run.   That being said, it is obviously not the only factor a good DC must weigh, when searching for "his" defense.
  2. What "fits" your kids?- This is much tougher than number one, and will be addressed later on in this article.  The key, to me, here is don't necessarily look at right now.  If you were like me, and had been at one place long enough to see a broad picture of the average talent base, year in and year out, then consider yourself lucky.  Most of us dont' get that opportunity, however all is not lost.  You can do a couple of things to get an idea of what your average talent level is where you coach.  These are:
    1. Talk to any former coaches where you are.  You'd be surprised how many still want to help, pick their brains and see what you get there.  Remember, they were here before you, and you can learn a lot from history, as history often repeats itself.
    2. Talk to long standing teachers or administration.  Basically the same as number one, yet probably with less insight into actual talent.  Still a viable resource though.
    3. Youth Coaches.  These are great, and they want to help so bad they can taste it.  Some can be super arrogant, but most just want to help the program.  See what they are seeing, as this is your window into the future.
    4. PE Coaches, at Junior High and Elementary Level.  These are the same as the youth coaches, but I think it's even better because they get a look at ALL the kids, not just the ones that play football.  As some of you know, little Johnny's mom won't let him play when he's 7, but when he's an amped up adolescent, she may kick his butt out to the gridiron!  These PE coaches get to see these kids daily, and should be able to tell you which ones are the good athletes or how each class should stack up, possibly giving you a window into what kind of talent base you have.
  3. What "fits" your coaching staff?  A very overlooked concept in my opinion.  You don't want to "out coach" your coaches!  Your assistants need to be able to, not only learn the system, but help you sell it as well.  If they don't know what you are trying to install or don't understand it, then you may lose them.  This is the first step in NOT selling the kids on the defense.  The players must see all the coaches "all-in" on what's being installed or your efforts will be futile.
Once you've picked the scheme, you can organize the scheme and package it in a way that you can teach it to your players.

Once you have a scheme in place, you now have to develop it.  You build a defense just like you do a house, from the ground up.  To start with, an often overlooked concept in football (and life) is communication.  Football players and coaches MUST be able to communicate.  The players must understand the language you are speaking.  Remember, just because you know what terms like "leverage" and "force" and "wrong-arm" mean, does not mean your kids will.  In some cases you will be teaching a new vocabulary for the kids to learn.  This will help both players and coaches  with communication.  What I do, in this part of the process, is begin with a scratch sheet of terms we plan to use, and then give them definitions.  If I think we can call something different, so that the players can better understand, then we do so.  For instance, at my last coaching gig, we were teaching the basics of the wrong-arm technique.  I was assisting the DL coach, and overheard a player say "Why are they teaching us this stuff, if it's the wrong way to do it?".  Funny to say the least, but hey, at least the kid was honest.  So we changed the name of the technique from wrong-arm to "splatter" (yes we stole this from the Scream and Splatter).  Anyhow, the kids understood that much better, and it also sounded better to them.  Another funny instance was we were using the words TAN and NAT for our tackle and nose stunts.  When I first started with this defense these worked as TAN meant the tackle slanted and nose looped, with NAT being the opposite (T before N, meant tackle before nose etc.).  For some odd reason, the kids kept forgetting or getting confused, so my "ever so elaborate" DL coach, changed them to TITS and NUTS.  Mothers didn't care for it, but the 16 year-old mind, ate it up!  We did not have any problems remembering our assignments after that!  Anyhow, the point here is, you have to base your defense around communication.  Communication is the top breakdown in, not only football, but in any organizational endeavour you may be involved in.  Without communication, your defense is just 11 bodies playing football as individuals.  Do not negate this step in developing your defense.

Kinda gets your attention doesn't it?

Once I start my scratch sheet of terminology, I keep it with me as I develop the names of the players that will play for me.  Some can be simple such as Nose, End Tackle etc.  However you name this, give the name a meaning or purpose.  Don't get too carried away with this concept either.  All you need to be able to do is tell a kid to "go play mike" and that player know, "oh, that's the middle linebacker".  These position names are usually one of the first items on my terminology list. 

Once you've got the position names, you now need the position attributes.  This can be a tough part, but it has to be done.  This is no different than if you were to start your own business and you give out job descriptions for your employee.  How can you possibly ask someone to be good at their job if YOU don't know what "it" is that you are asking them to do?  No different in the game of football (remember the blog title is Football is Life!).  You need to know what you are going to ask of each position.  What is each positions job description?  Don't tie these things down to player attributes, especially if you are in the high school game or even lower levels.  You can't recruit, and you have to just accept what you get, so tying a position down to player attributes is senseless.  I've been down this road before, and all it does is give you a headache when you find out over half of your players don't fit your physical criteria.  Use this list you develop to search out and locate players that will have the potential to do what is being asked of them by the scheme.  This is the first step in putting the square peg in the square hole.

You want to avoid this situation if possible.


Once you've labeled the positions, and described their duties to the defense, now you can get down to the technical aspect of building your defense.  Like I always say, start from the ground up.  The first thing I look at is the fronts.  More specifically, how am I going to call the front?  "13" is much shorter than "Eagle Zipper Hero", and a whole heck of a lot easier to shout out in the final two minutes of that illustrious State Championship game we all want to coach in.  Keep things as simple as possible.  I have used both numbers and names over the years to call my fronts, and have been the happiest with numbers to set my front.  Obviously whatever you call your fronts needs to be added to your terminology list as you go.  In the background you need to be thinking "How can I signal/communicate this to my players?".  It's all fine and dandy to sit at the dining room table and think, yeah Bear Foxtrot is a great name for this front and blitz, but how the hell are your going to signal that in, especially if you choose not to use wristbands?  Keep these things in mind when deciding what to call your fronts.  Again, all roads lead back to communication.  As always the K.I.S.S. (Keep It Simple Stupid) method applies here.

Wonder if they got the call right?

One other item in calling the front is "tags".  Just like offensive coordinators (OC's) use route tags or blocking tags to change up an assignment, DC's can do the same.  A "tight" call in our defense means that no matter what the offense comes out in, both our defensive ends (DE's) will be in 5 techniques.  So the call "31 Tight" puts the front in a 3 weak, and 1 strong, with both DE's in 5 techniques.  Tags should definitely be one word, and more importantly one syllable words.  Again, you don't want to have to say "13 Rover Slide, when you could just say "13 Slide".  Tags are a must though, as they help adjust and tweak your defense to be multiple (which will be discussed later).

So to summarize, keep front names, short, logical and easy to communicate.  You do this and you are well on your way to developing a rock solid defense.


Once your front has been establish, now it's time to put the roof on the house!  The secondary is a vital important part of your defense, as it determines your front's alignment (thank you brophy, if you haven't checked out his site go here immediately after reading).  Most coverages are called with a number, I prefer colors or tags to calling things as it lends itself to being more mysterious to figure out what we are doing for the offense.  When you stand there on the sideline holding up the number 2, it isn't too hard to figure out what coverage you are going to be in.  Again, just like the front, keep it simple and easy to communicate.  Now I know what you are thinking "Duece how many coverages should I run?".  Well, I can say this, I've run as little as 2, and as many as 6 in my years as a DC.  It all depends on your situation, but over the years I've averaged probably 4 coverages per season.  I feel, in the secondary, you need the following:
  1. Man Coverage- I think you need some form of man coverage.  I taught cover 0, for blitzing purposes.  Man coverage is not hard to teach, man coverage adjustments can be tricky, but let's be honest, when your in man, it's street ball baby, go guard your dude!
  2. Base Zone Coverage- Whatever coverage you decide to base out of, this is it.  This needs to be a coverage you are comfortable teaching and know inside and out.  It needs to be a coverage that has adjustments for just about every situation (and I DON'T mean checking to another coverage).  For me, as most know, this is 2 read or Blue coverage (for more on Blue coverage go here to read up).
  3. Alternate Zone Coverage- I think most of us agree, it's hard to sit in just one zone coverage all night and be totally sound.  This second coverage for me has been a mix up between squat halves cover 2 and "country" cover 3 (vassdiddy's term), with the latter being my most popular choice.  Whatever your second choice is, make sure it meshes well with your first coverage.  Try to package these two coverages so that there is a lot of overlapping teaching and more importantly overlapping technique. 
That's basically it, for picking your coverages.  Now, let me add something here too.  If you look at my playbook, you will see over 10 coverages in there.  Holy cow!  Now, I always look at a playbook like my tackle box when I'm fishing.  I may have 10 lures in my tackle box, but on any given day, I only use two or three of these lures.  However, I have those other lures there, for just in case that special situation comes up that I have to use it.  You don't have to teach all these coverages, and if you hand out playbooks, don't put these additional items (this goes for fronts too) in there.  Remember, there is no crime for planning ahead, and there is no problem with having too much in your playbook, so long as you know when and what you are teaching. 

Goal line

Every good defense needs a goal line package.  My advice here is, keep it as simple as possible so your kids can play to their full aggressiveness.  I usually choose to reduce the front and walk up OLB's or safeties closer to the LOS.  However you choose to do it, keep it very simple and easy to get into and out of.  When selecting coverages, keep it simple here too.  I've done both zone and man, and both have their merits.  The key here is, minimize the adjustments, and keep your defense's ability to play fast at a premium. 


Now that a "base" has been established you can now look at how you plan on moving and attacking with your defense.  I usually start with stunts, because stunts can be stand-alone items or coupled with blitzes.  Stunts are no different than any other part of  your defense.  These need to have terminology that is "user-friendly" and can easily be coupled with the front call and maybe a blitz call to communicated the play at hand.  You don't want to be stating "Stack TAN Atlanta Bullets Away", when you could easily call 13 Bullets A instead.  The naming of the play is ever so critical in the communication process, that you need not overlook it when developing your defense.  The "nuts and bolts" of both stunts and blitzes are tags and codewords that move players to either certain areas or gaps within the offense.  How you choose to communicate this is vital to the success of your defense.  When looking at stunts, figure out a couple of things before putting pen to paper:

  1. Can my players do what I'm asking?- No need to develop a base defense around your players, only to turn around and have them be asked to run stunts that they simply cannot do.  The longstick looks good on paper, however if your DE simply cannot execute this technique what do you do?  Do your fire the DE for his backup?  I think not.  You need to couple the stunts with the same philosophy on why you chose your defense in the first place.
  2. Will I actually use or need the stunt?- This is a serious dilemma, as in our minds at the desk in our office, sure we are going to feel the need for the stunt.  When in actuality you may never use the darn thing.  Again, there's nothing wrong with having it in the playbook, but you can save a ton of time and energy eliminating stunts you feel you will not need or use.

This is probably NOT a good stunt!


Blitzes are no different than stunts, basically speaking.  You need a way to get certain players to attack certain gaps, and be able to communicate this effectively and in a timely manner.  The rules for blitzes are the same, because to be honest, you can draw up a boat load of blitzes, to find you only use a handful of them in a single season.  Again, no penalty for having them, but how many of us have the time to draw up 50 blitzes?  A couple of items, in addition to the ones listed for stunts to consider are:

  1. How will the blitz adjust to certain formations?- The worst thing you can do as DC, in my opinion, is have your defense check out of a blitz all together.  If you can only run a blitz to certain formations, I say don't use it.  You will need to build in some "fool-proof" adjustments so that your players can make these on the move, and then still bring aggressive pressure (remember the old adage, "paralysis, by analysis").  No need to have players moving and thinking too much, as this simply slows down your pressure package and plays right into the hands of what your opponent's offense is doing.  I know I keep beating a dead horse here, but these adjustments must be able to be communicated quickly so that the defenders can adjust, realized their new or altered assignment and get back to what it is that you were intending to do...BRING PRESHA (stole that from brophy too, damn, I'm such a thief)!  Don't use "Check Razor" when "switch" will do.  Remember, in football, "less is more".
  2. Build in a "bluff" system.- This is important to me, because I've found it's very difficult to get the high school and lower level kids to not show their blitz intentions.  The other thing is if you don't show, sometimes your guys don't get to their intended target.  The easiest thing to do is to "bluff" your look.  You can do this numerous ways, and the way I do it, is simply call "show" before the blitz name.  This means we are NOT blitzing, yet "showing" the blitz and backing out at the last minute.  This is a very simple change up, and will keep opposing offenses on their toes, especially if you are a pressure based defense.
Blitz Coverage

When developing your pressure package, one item you need to look at is, how am I going to cover it?  The traditional manner is play cover 0 or cover 1 (man) and get after it.  Absolutely nothing wrong with this, and I even based out of this a year ago myself, however don't knock zone blitzing either.  Zone blitzing is a great technique, that has numerous advantages over it's man-to-man counterpart.  Taking a look at the pros and cons of each:

Man Coverage- Pros
  • Simple- Adjusts to everything, everybody has a clear cut assignment.
  • Works for every blitz- With simple adjustments man coverage works with just about any blitz you can draw up.
  • Can send more than 5- Zone blitzes are usually regulated to sending 5 and dropping 6, whereas you can send as many as you want so long as you can cover the eligibles you are good.
Man Coverage- Cons
  • Matchup issues- If you're not careful you can end up with serious matchup issues by having a LB covering a slot receiver, or simple matchup problems where their Jimmy is better than your Joe.  Either way, man coverage is very limited to the matchups you can bring to the table.  The more of these in your favor, the better off you are in man coverage.
  • Not all 11 eyes on the ball- DB's that are in coverage are not watching the football, thereby reducing the amount of eyes you have on the football.  This is why screens and draws are effective methods of hurting man blitz schemes.  Also, these schemes can be deadly if a run play breaks the second level of the defense, as the third level has its eyes on their man.
  • Motion/Shift Confusion- Offenses that find you in man coverage will test your defenses ability to adjust by motion or shifting.  This can confuse man defenders and result in two defenders covering one receiver.  Not hard to work on, but when it happens, it's usually six!
  • Not a good coverage vs. the option- The man-to-man defense is severely limited in attacking option schemes as run support can be reduced by simply having a slot receiver run a route, thereby running off the dedicated force player.  For this reason, man coverage teams have a tough time pressuring the option schemes.

Zone Coverage- Pros
  • All 11 eyes on the football- Great at stopping both the run and the pass.  Zone blitzing severely negates the draw and screen game offenses use to beat pressure schemes.
  • No Motion/Shift Confusion- As long as the underneath droppers can understand where they are supposed to be, and don't lose leverage on the formation, you are sound as a pound, and if one of them does bust an assignment, you still have three deep zone defenders behind them to cover up the mess.  This is not the case in man coverage.
  • Sound against the option- With all 11 attacking the option, and still having two dedicated force players to either side of the offense, you can actually bring pressure against option attacks.
Zone coverage- Cons
  • Holes in the zones- It's still zone coverage, and as we all know, there are holes in it.  Man coverage covers up these issues, however the good thing is in the zone coverage you have defenders there to make a tackle , that might not otherwise be there in man coverage.
  • Matchup issues- Some zone schemes involve dropping your DL as a middle dropper, or the #2 dropper into the boundary.  This can pose certain matchup problems if this defender is not an adequate enough athlete to do so.
  • Adjustment issues- A good zone blitz scheme must be able to adjust to multiple formations.  You don't want to get caught with a DE dropping to the field, or a nose as the #2 dropper, when he should be the #3 dropper.  It is not impossible to create these adjustments, but it does take some practice, work and good communication to stay sound vs. the myriad of formations/motions/shifts you may see throughout and entire football season.
  • Practice time- In my experience, zone blitzing takes much more work and practice time to be effective.  Not knocking it, as I love the zone blitz it involves more people that are not used to covering (DL) than man coverage does.

No matter how you choose to pressure the offense, keep it simple, clear and concise.  Players cannot react and get after the offense if they are constantly having to think, or are in a state of confusion.  The key here is "less is more".  When you start devising these blitz and stunt schemes you will realize that the possibilities are endless.  Well, endless, can lead to win less, especially if you try to install all of the potential blitzes and stunts you come up with.  Think about blitzes and stunts that would work well in your league.  Look at ways to draw up one blitz, but maybe draw it up four ways with a LB attacking a different gap on each blitz, but the DL stunting the same.  This keeps teaching to a minimum, by only altering what one player is doing.  I call this "multiplicity through simplicity" and it has worked well for me over the years (go here to see some of what I did with TCU's blitz package).

Special Defenses

No matter what league you play in, or what level, you are always going to need some "special" tweaks to your defense in order to handle certain things you will see throughout the course of a season.  This may be no more than a nickel or dime package to handle end of the game situations or two-minute defensive situations, but these need to be planned for and located somewhere in your playbook.  However you choose to do this, whether it be with base personnel, or if you decide to switch to nickel or dime, write out how you plan on doing this.  Some things to ask are:

  • How am I going to get into this special defense?- Whether it's a personnel change or a major coverage change you need to be prepared.  Think how you can communicate this in one of the most heated environments in all of football...the two minute drill.
  • If you go to nickel or dime personnel, how are you going to get into or out of it?- You need a clear cut defined system for getting into and out of personnel groupings.  What do you do if during the two minute drill, the offense is faced with a third-and-one?  Your defense needs a good system for getting players into the right places and in a timely manner.  Plan this out in the off-season, so you are not scrambling with 1:22 left to go in the fourth quarter of a district or playoff game!
  • Why am I getting into this defense, and can I do without switching personnel?- This question should've been at the top of the list, as it is a very important one to ask.  I based out of nickel personnel a year ago, and I had to ask myself in goal line situations, should I have another personnel package to handle that?  I felt like I could get it done with the guys we had, so we did not switch, however in some cases we could have used this "heavy" or "jumbo" package to help us out.  If you can do it without switching, you've simplified your defense in numerous ways, however most of  us are not blessed with this kind of "every down" talent, and need to have adjustments.  If you can eliminate this adjustment, you have less specialized teaching ahead of you, which makes for less "moving parts" in your defense.

Special/Additional Fronts/Coverages

Some offenses require that extra little tool or weapon you need to make their lives miserable.  This could be anything from a 6-1 look to defend the Flexbone, to run the 4-3 Lightning vs. the Wing-T.  Now I'm not saying you need these packages, but in some cases these additional items can be that extra bullet in the gun you need to attack your opponent's offense.  When looking at these additional defensive packages, here are some thoughts you should weigh on before adding:

  • Will I use this package?- For years I had a stunt package that was in my defense for the Wing-T.  We used twice in one year, and then did not use if for four years after that.  If that's the case, save your paper and print that part of the playbook out when and if you ever decide to use that particular package.  No need giving your players a bunch of stuff to look over they are not going to use, remember "less is more".
  • Add this package to YOUR playbook!- What I mean here is, if you have to convert terminology, do so.  Don't add language or terms that may already be in your playbook.  Unnecessary verbiage is the first step in the demise of the communication process in your defense. 
  • Don't have multiple packages that do the same job.- Before I knew it, as a DC, I had three or four ways to defend the Flexbone.  I also had a similar issue for the Wing-T and Double Wing.  Find a package you like, and that fits your kids, and use that.  Try to incorporate it from year to year, so there is less teaching time involved on a yearly basis.  This allows this special package to become more of an adjustment than a special defense.
  • Don't use a completely different defense to do what your defense should be built to do.- This is key.  Don't get into the 3-4 from the 4-4 because you heard it was the "best" front against the triple option.  To be quite honest, there is no BEST front out there, they are all good defenses with their own set of strengths and weaknesses.  Even the 46 can defend the option (as some people say it can't) just like cover 3 WILL work against the spread (if executed properly).  Build things INTO your defense.  As was once stated on the Huey board "play defense, not defenses".


Remember that scratch list at the beginning of the article?  Well, time to refine it.  Once you have developed your terms, alphabetize them, and then review them (alphabetizing helps you find them easier when editing).  Could you get away with less?  Do you have terms that are similar in sound, but dissimilar in meaning?  Do you have terms that are being used for basically the same thing?  Try to trim your list down as much as possible.  Once you have done this step, now look and see if you can find synonyms to the words you have chosen that might be shorter, or "fit" better than the original word you used.  Also, see if you can find a word that's easier to communicate than your original word you came up with.  "31" is a heap more simple to communicate than "Eagle" or "Under".  Some may argue, but try finding a hand signal for eagle or under.  Also, if using wristbands the number 31 obviously takes up less room than eagle or under as well. 

I'm beginning to think this to be true...


If you are signaling in your defense with hand signals, make sure to make calls that will fit.  This needs to be a tedious process, as you can find yourself in a world of trouble if you can't signal in your defense.  You need to have signals that relate to what you are trying to communicate.  Try to make these calls as dissimilar to each other as possible when developing these.  This way there is no confusion as to what you are calling.

I've used wristbands, and this method of signalling has it's merits and drawbacks.  Make sure the players can read the wristband underneath those Friday night lights.  They DO look different in an outdoor setting compared to what they look like in the light of your home or office.  Prepare for this, because the worst case is you not being able to signal to your defense what you need them to do.  I also recommend NOT using numbers for coverages if you use numbers to delineate the calls on your wristband (yes, I actually did this one year, and it was a major cluster trying to check fronts and coverages).  I recommend using names or colors for your coverages if this is how you plan on signalling in your defense.

However you decide to do the on the field communication, write down how you plan to do it.  If you are going to do hand signals, then write them down, and describe them.  If you use wristbands, write up a plan for creating and editing these wristbands. 

A side note with wristbands, is to have a backup plan if the bands fail.  If the kids can't read them, or they get left at home for an away game, have a backup plan so you can still effectively signal in your defense.  Believe me, this CAN and WILL happen, and you NEED to be prepared for it!

"Eagle Zipper Hero" favorite!

Other Recommendations

Some simple things I've compiled that I recommend for every DC is as follows.  This list is not an end-all-be-all, however I think it's a very good guide to go by, and that's why I'm sharing!

  • Whatever front you run, have a way to get into a different look.  A defense that is multiple, is much better than one that is overly simple.
    • If you are an odd front, have a call that puts you in an even front look, and vice versa.  Many offenses have rules based on odd and even fronts, or covered and uncovered linemen.  By mixing this up, you test the offenses ability to adjust and adapt to what you are doing.
  • The same is true for coverages, have a way to go from middle of the field open (MOFO) coverages to middle of the field closed coverages (MOFC).
    • Even better, be able to disguise this look.  However the basis here is, if you are a 2 high safety structure, have a way to play 1 high coverages and vice versa. 
  • Don't add it if you don't need it.  Many of us WANT to run certain fronts, stunts, and coverages, but you have to ask yourself do I really NEED it? 
  • Don't run it if your player can't play it.  Similar to the above, but even more defined.  If your DB's struggle with zone coverage, more than likely man to man is not necessarily the answer.  This is a tough one too, because often times, it doesn't matter what you run, your players are simply not athletic enough to run either.  The main question then is, which of these CAN my players do best.  Once you find the answer to that, then go from there.
The latter bullet, got me to thinking too, don't look at what you CAN'T do, look at what you CAN do.  This is very important for more reasons than one.  First, the human mind associates the word "can't" with negativity.  As a coach, you can ill afford to have negativity brought into your defense.  However if your mind is in the negative, your players will soon follow.  Look at things in terms of that what your players can do.  Secondly, every defense CAN do something well.  Even if you have to list the "can's" and "cannots" of your players, do so.  This will give you a glimpse into how you can blueprint a defense around your players strengths, thereby minimizing their weaknesses. 

This article is not meant to "cookbook" defensive football.  There is no standard recipe for success.  What there is though are some very valid points to look over and address when installing your defense.  I hope this has touched on these points and been helpful in some way.  Keep the comments and the emails coming, and hopefully some of this stuff will lead to some success for you on the field.

Good luck!