Defense has always been my strong point. I can shut anyone who doesn't rocket catch down (although that is becoming easier to stop). All you need to remember is to adjust to what your opponents is doing. Here are some of my tips

1) Disguise the blitz. If you blitz, it's vital that you get to the QB. If he knows it's coming, usually he can audible into something that will tear your D apart. Those defensive audibles are there for a reason, use them. And use them at the perfect time; right before the ball is snapped.
2) Use the Multiple D playbook. That will give you both the 3-4 and the 4-3 to work with. Use the 4-3 if he's running the ball inside, and the 3-4 if he's pitching tosses left and right.
3) Play with your safety to make you sure absolutely shut down the run. To win, you must force your opponent into obvious passing situations.
4) Like I said, adjust to what he's doing. If he's hitting short slants, screens, and RBs in the flats consistently, use the cover 2, but you must get pressure on him with your front four.
5) If he's running around like a b!tch instead of standing in the pocket, audible your two DE's to hit the outside as opposed to rushing up the middle, and have one of your DT's spy on the QB to make sure he doesn't take off and run.
6) If he's consistently throwing long bombs, get in your dime package and use the cover 4. Again, make sure your DE's hit the outside to pressure the crap out of him. Throw in a Strike 2 Deep blitz every once in a while to put major pressure on him. Then use your MLB to hit the zone. This will result in a sack or interception almost every time since he's not taking the blitzers into consideration.
7) If your opponent is being a douche, and constantly rocket catching with his best WR, then you're going to run into problems. Use your safety and simply try to pick it off yourself, since that's the only way you're going to stop it.
8) Utilize the press coverage and soft coverage to your advantage. Sometimes on third and long I like to use the cover two, and back the secondary off. This will make him think he can hit someone underneath. The only time to press your DBs is on short yardage situations. Other than that, don't press them because the WR will always get behind the DB, regardless of the zone you're using.

Anyway, I could go on forever and get into defensive hot routes & other tricky schemes, but you should get the hang of it over time. I don't even call plays at the main screen. I wait to see what package he's using, then go quickly to whichever defensive package is necessary (it messes with his head)... Then I merely audible to the play I want while on the field. The key to being successful on defense is simply tricking your opponent...

2 RB 1 TE 2 WR = 4-3 or 3-4
3WR = Nickel
4+WR = Dime, with four DL
etc.

Anyway, that's my theory and I'm stickin' to it