Prepare adequately for your performance. If you are performing from memory, make sure you've had the piece you're performing memorized for some time. Set "memory guideposts" by practicing starting from a few predetermined points on every page. This way, even if you mess up or forget, you can always start somewhere close by.
Swing your hands together and clap sharply in front of your self. Do the same behind your back. Repeat this several times. The centrifugal force as well as the impact of your hands will force blood into your hands and aid in stopping shaky hands. Do not clap your hands overly vigorously. You don't want to hurt yourself.
Rapidly shake your right arm for a count of ten. Do the same with your left and with both of your feet. Like the previous tactic, this will force blood into the extremities of your body and will help stop quaking.
If you're playing more than one piece, play first the piece you're most comfortable with. This will put your mind at ease, warm up your hands and help you get into a flow.