As others have said, Gatorade is awesome. I've been bringing one to gigs for some time now, and if it's an outdoor gig I'll bring a few. I have also kept a small fan in my gear box for years now, it tilts, and move enuf air to make a difference.
I try and wear short sleeved button up style shirts rather than tshirts, I find them to be cooler. Shorts if possible. Indoor or out, in the hot summer months.
I also read about milk being better at hydrating than water, tho I would never drink it at a gig. But the day/night before, and earlier the day of, if possible.
As for alcohol...well...I know it's best not to. In most cases this is true, let alone hot outdoor gigs. But damn, a cold beer or twisted tea is awfully good on a hot day. I still keep the Gatorade going, too. (It helps, anytime I'm drinking.) I usually try to hold off till at least halfway thru, and only have one till the show is over. And not every time, either.
What I keep forgetting is to bring a towel! I've played some gigs where friends there took small towels and dunked them in cold water, and gave them to me to put on my neck, in between sets, and even to wipe my face between songs. That's something to think about having on hand if it's gonna be a particularly hot day.