I always go to the bathroom and make sure I'm empty. When I get nervous it goes straight to the lower functions. I also make sure my costume is firmly fastened on. I've had some costume malfunctions in the past and I never live them down.
I used to sing folk music in a little coffee bar in. Brattleboro. I was on stage for years-- WAY off Broadway-- doing classic musicals, but was never nervous. I would always whisper a little prayer, though.