The main object in playing defense... be between the opponent and the goal. And preferably the opponent and his options (to pass or whatever).
Always guide him outside -- make sure the easiest way for him to go is towards the sidelines, and not towards the middle of the field.
I made my mark with the slide tackle in high school, and it can be a thing of beauty... just make sure to get the ball first, so there are no penalties.
And to echo what's been said a little differently, always be on the move. It takes more time to get up to speed when you're standing still than when you're heading in the wrong direction, believe it or not.
And for those fancy pants who think you can't have fun on defense, I have scored on defense, and it's just as much fun. And it's more impressive when you realize how far away you might be...
