These suggestions were actually formulated with football in mind, but with a little thought they can be used in almost any sport.
When someone is coming at you, to tackle you, or to get the ball, bar your teeth, hold out your hands like you're going to claw them, and hiss.
Perform grounding rituals in the middle of the game
Replace the ball with a decaying turkey roll (Turkey Napalm!!!)
Sit/stand in the middle of the field, staring at the ground. (A blade of grass works well) start screaming for no reason.
Start screaming "we're all gonna die!" and/or "There's a sniper in that tree!!!"
Run up to the person with the ball and kiss them suddenly (be careful who you try this on)
Be a mime. Play with an imaginary ball.
Eat the ball.