Chin jab. Use your full bodyweight and hit BG on the point of the chin with an open palm. Not safe at all for sparring. Possibly lethal move. This is why boxers typically tuck their chins in.
The move with a hundred names - when your adversary is on the ground, jump onto his upper chest. When just above the guy (in the air), shoot your feet forward, heels first, and pwn him. Very good chances of the guy ending up dead. Obviously, this isn't good for sparring either, and being a "finishing off" move, comes with a host of legal issues. There are few situations where this method would be acceptable, but it is very effective.
Kick with the side of your foot, not the toes.
Karate chops to the neck/throat are fun. Hit an inch below the Adam's apple for added damage (again, not for sparring).
Work on a few techniques, and know them thoroughly.
Go on the offensive.
Win.