This is why Madden is the best game ever... Very few people get into what they should do on BOTH sides of the ball. In my experience, I've found a few things to be helpful when I was learning to play defense:
1) User Control
The toughest part about playing defense is reacting to the unknown. You won't have time to remember where the swat button is when you need to swat a deep pass. Like real football, this technique must be instinctive. The less you have to think about which button to press the quicker you can actually press the button. Master your player.
2) Augment coverage
Just because you call a blitz doesn't mean you have to rush everyone that's supposed to rush. Controlling a blitzing player and freelancing is a good way to defend without screwing up your coverage. If you need to switch away, he'll go after the ball carrier.
3) Switch when needed
Knowing when to switch men is an art form. Sometimes you'll be better at making the play, but it's unlikely until you master #1. If you can make the play without switching defenders, do so. Resist the urge to do everything until you are 100% comfortable taking action...
I prefer to play defense to offense, but you can use what you know about one to attack the other. There's very few things that feel better than a USER PICK that you take to the house.
Hope this helps...
Later