Sleep right the night before, eat right throughout the day but leave it a good 40mins without food before you're set, exercise in the day time before hand, something like 40 mins cardio/20 mins free weights and if you've got to sing no smoking beforehand and run your full set at home prior on the day of the gig. That's my typical routine, oh and a I have 500ml energy drinks one for the drive there and one to get me home. Oh and bring a book if its an early sound check, or a pillow if you're the kinda person happy to catch a few winks while your waiting. Xx