Keep players in position to defend. If all of your players are attacking the goal, you're going to get burned when the opposition goes the other way. Always keep some defenders back, unless your defensive strategy consists of giving your goalkeeper an intense workout. A formation that effectively matches the team you are playing is essential. Try to ensure you have a player to adequately defend all of the attacking players.
Nothing is as satisfying as a good tackle.
For the most part, a defender should go for a conservative tackle. However, there are times when you'll want to crank up the pressure with an aggressive tackle. Be aware, though, you could draw a yellow card, or worse, land yourself an early bath. Know when to keep your distance. If a player is constantly up on his opponent, he's inviting trouble.