I echo what others have said. It's more rock and roll to drink during gigs, but I learned the hard way, that having a bottle of whisky next to me during gigs was a mistake. Alcohol dehydrates, and saps your get up and go.
These days, lots and lots of water before and during the gig. I'm the lead singer/Bass player/Front man, and it takes a lot out of you to perform a gig for an hour and a half, and hump the gear in and out, before and after the gig. I find it physically very tough, but then I'm getting old too.
Another important bit of advice I got from someone in the know, was to eat, and eat [i]plenty[/i] before the gig, carbs carbs carbs.
Gigs are workouts for me, and I sweat like a glass blowers arse too, so I have to fuel up or crash and burn.