A power-nap if I can get one - ten to fifteen minutes is enough. Lot's of stretching before the show. Good quality meal with as little processed sugar as possible. If  I'm really tired I'm careful not to eat too much heavy carb (pasta, bread) because these tend to make you feel even more drowsy.  Trying to eliminate unwanted stress and tension in the muscles (playing or not) alleviating some energy loss. Try to get into the emotional side of the music while on stage - emotional highs can get you through the worst physical fatigue.