I just did Badminton last week. Keep it as close to the net as you can, just drop it over then when your oponent has to hit it high to keep it up and over, SMASH IT!
I'm not even going to read everyone elses reponses - the best tip is just to throw your racket at the other guy when he's to busy looking up in preparation to hit it. Or just tackle him, punch him in the throat and steal his car. You've won FAR MORE than on court, my friend.