This will deal mostly with the ways the defensive AI in Fbpro 95 (the way I see it) is unlike reality and how to deal with it. Most of the problems stem from the fact that fbpro players have no sense of responsibility or team play. They all play as individuals. I know fbpro has the best game on the market, and the AI is hard to program, but it still sucks.
The DL are as dumb as the OL. In reality a DL explodes forward while making an automatic read on the OL as to whether the play is a pass or run A rush read calls for the DL to pursue the ball-carrier while running parallel to the LOS. They must keep themselves evenly spaced to prevent any holes from opening up that would allow the RB to cut back up. In addition, one or both the DEs may be given contain responsiblity depending on whether an OLB has the job or not. If they read pass, the DL collapse the pocket while keeping their lanes so the QB can't sneak it. Fbpro pretty much leaves you with the decision to sit around at the line of scrimmage no matter what happens or go after the ball-carrier with reckless abandon. For some reason, fbpro chooses to give DL with "move to" logic a late start, so DL stunts with "move to" aren't really worth it; you should line the DL up exactly where you want them to go. Since "run rush conservative" doesn't seem to help much better than "run rush aggressive" against the run, and it'd be inane to leave yourself without a pass rush, you're pretty much stuck with the choice of "pass rush" or "run rush aggressive." (I haven't tried out run rush balanced in a long time, but I think I remember it not having an effective pass rush either) Given that, I haven't seen the pass rush logic do very well against the run, and I don't like the way the pass rush logic pass rushes either. It makes the DL head straight for the QB, regardless of what's in their way. With run aggressive logic, they take a couple steps straight forward, then head for the ball-carrier. That method more often fools and avoids the OL.
Now where do we put the DL? I like to put a DE with good AC way outside each tackle, almost until he doesn't go into his 4-point stance anymore. For run defense, the DE provides a first line of defense for the pitch, a far more dangerous weapon for the offense than inside rushes (because he's way outside the tackle, he can often avoid the blocker in an unrealistically effective technique the good teams in your leagues often use. They try to run a strong blocker at the DE, and if the defense is playing standard M2M, the defender that was on the blocker continues to run off for some reason in the direction the blocker initially ran toward the DE.) And if the DE has real good AC and AG, he can often run around the tackle and catch the ball-carrier in an inside run from behind for a stuff. For pass rushing, the DE spreads the OL out, creating some room for the others to get at the QB, while usually creating a one-on-one situation with the tackle. If the DE can get at the QB, fumbles are a likely result because of the angle he's coming at. (Finally! Something realistic!) And if the QB rolls out, the DE can often snag him.
I like to have two DTs to muddle the middle for inside runs. With some push and penetration, the ball-carrier often slows down to a crawl, confused on where to go. Then your defensive teammates come help out and it's over. The DTs can sometimes break through on pass rushes for a sack, but not often. Another thing you might try is lining the DTs on the OTs so the DEs have a clear shot at the QB. But the QB when trying to pass often pulls the "drop-dead" routine on the DEs , and runs up the gut for a considerable gain. Experiment with gaps you want to put the DT's into, perhaps drawing double-teams from the OL to allow LBs and DBs to blitz.
The LBs and DBs are as dumb as the DLs. In reality, the LB first reads run or pass, and then looks at one or more keys (pulling guards, false steps and movement of RBs, etc.) to usually take him right where the ball is. (Sierra doesn't even know what a key is as evidenced by the "key on quarterback" and "key on hot player" logic) Although the NFL more often employs base blocking schemes than pulling ones, a pulling near-side guard or tackle is a very effective technique for outside runs in fbpro since the defense pays no attention to him. In fbpro, a LB or DB with read logic will immediately make a break for the QB if they read run. For this reason, don't line defenders with read logic up near the middle of the field and expect them to help out on the outside. However, lining them up a decent distance back from the LOS is THE best way protect against the inside run. A defender lined up close to the LOS will read run, break toward the QB, and often run past the ball-carrier before he realizes the ball-carrier has the ball. Also, for some inane reason, the computer gives defenders lined up at least 7-9 yards back a quicker read than those lined up closer; the defenders 7-9 yards back can get to the LOS as fast as the RBs can, and the defenders even have the opportunity to react to where the ball's going. For this reason, line up 2 or 3 safeties and LBs 7-9 yards back to stop the inside run. They'll provide a handy 2-deep or 3-deep coverage 15 yards back if it's a pass.
There are many things you can do on pass coverage, but what you absolutely must NOT do is play a normal M2M. You'll get destroyed by outside runs and the silly "throw a fake and let a fellow WR run away un-guarded" nonsense. And the best way to elimate the other team's outside running game is to play close bump. To prevent the too-often called holding penalties associated with it, you can give the defender "stop for 0.0 seconds" logic to let the receiver run right by, something that is perfectly safe- even beneficial for getting more interceptions- if you have deep help. The only big problem with this method is if the offense uses motion to outrun the defender. Without bump, there are other ways to get under the receiver using the "shade under" logic with few penalties, depending on the defender's ratings. Since the players' ratings change during the game, though, that isn't as certain to work. If you want to learn about zone coverage, read Cory Ridgway's Coaches Clinic document. An effective pass rush can be made with only 5 defensive players by using the previously-mentioned DTs to gather double-teams and have someone come from behind. It's best to use run defensive aggressive logic for the 5th man for the reasons already given. There's no purpose in disguising the blitz; after all, the offense is as dumb as the defense. Just try to get the 5th man to the QB as soon as possible so the RB's don't have much of a chance at reacting for a block.
Looking at the way fbpro handles personnel, it can be generalized that DBs are much better athletes than LBs in this game. The point of having LBs is for their strength, but many DBs' strengh potential is right up there with the LBs', while fairing much better in all the other athletic qualities. So draft and trade for lots of DBs, train their ST, and play them as replacements for LBs. However, fbpro requires 5 LB spots, so try and keep 2 good LBs to use regularly so the DBs, of which there are a limited number, aren't drained. The same can be said of TE's and their ST advantage over WRs.