THIS^^^^
Truth be told, I can rarely make this happen. But I am a singer as much as I am a bass player, so I can't eat a big meal before a gig... a stuffed belly prevents using the diaphragm to full avantage, and I hate having to gulp air when I sing.
Sushi gives you a major protein rush, plus the carbs from the rice for energy, but doesn't bulk up your gut (unless you have a ridiculous amount of money and no self-control).
Problem is, it's pretty rare that I can score some sushi in proximity to where we're playing. So I often compromise with a turkey sandwich from Subway with all of the veggies, banana peppers and vinegar, and no oil.
Caffeine, energy drinks... NEVER! In my 30+ years of playing, I've tried this a couple of times when I was doing a Friday night gig after a long day at work, and each time I got shaky and my muscles in my hands got tight and I couldn't play well and my voice sounded crap.
If I go into a gig fresh and nervous (yes, I still get nerves), I'll do a shot before we start. Then I typically nurse one beer per 1-hour set... although I often forget to stop to sip anything while I'm playing. I tend to get into the moment.
Funny, I play country... I guess I oughta be eatin barbecue and sluggin Jack....