Make sure you're not late to any matches. If time is called when you're late to a match, you automatically lose. I almost won a game this way last year. Another 43 seconds and I would of, but I took all 6 prizes first.
Watch your opponent carefully. If you suspect something just happened unfairly, tell them to stop whatever they are doing while you hail a judge. Don't be afraid to call somebody out on something.
Socially, play nicely and be friendly. But tactically, play hard-ball. If your opponent asks to take something back, your general answer should be "No, sorry."
Generally you'll have one person on offense during a game, and one person on defense. One person has the sword, the other the shield. Now, which most often wins in a fight? Always stay on offense.