You ae most likely dehydrated. That's what usually happens whith cramps. And drinking beers only makes it worse. Stretching after the gig would also help. Sports drinks like gatorade help. Drink during the gig before you get thirsty. Every 10 minutes or so.

Lactic acid build up along w/dehydration is the cause. I suffered this problem for many years as a child. I played football for 12 years and from the time I was 10 until around 26 they were a regular, sometimes nightly occurance. Eat a banana or two and drink plenty of water before you gig. The potassium helps to aleviate the lactic acid build up.

Mine were so severe that I would miss school on occasion because it hurt soo bad. Even after football it continued because I was went into championship beer bonging in my college years. The only thing I could do to get relief was eat bananas before my long nights of partying....

Good suggestion Jstix, I forgot about bananas.

Drink some tonic water. The quinine in it relieves the spasms,