I live in Central Texas (Austin), and this was written based on my experiences here, but it has relevance for just about anywhere. Summer here typically means temperatures in the mid 90's and up. It is important to make sure you do not get heat exhaustion, or worse, end up in the hospital with heat stroke.
Chill a couple of plastic water bottles in the fridge the night before and pop them into a small ice chest with some ice. If that supply is not enough, bring an extra bottle with a wide mouth and/or a plastic glass you can add ice to. Drink, drink, drink! Gatorade or other electrolyte replenisher is good too, but mostly drink plain old water.
Bring a bandana you can wet with ice water and put around your neck, it is amazing how much it will help keep you cool. Also, bring a small towel you can wet with ice water to wash your face, hands, feet, etc.
Bring a couple of fans, one for yourself and one for your customers. If there's no electricity available, buy some small battery operated camping fans at Academy, one for you and at least a couple to set on your tables to blow on your customers while they're browsing. They'll appreciate it and will stay in your space longer. I also hang a couple from the tent structure to keep the air circulating.
If you are willing to spend a little extra, you can lower the temperature in your tent by 10-15 degrees by creating a "ceiling" with shade cloth. It can be purchased in the garden department of Home Depot or Lowes for approx. $30 for a 6' x 20' roll. Side panels made of shade cloth can also be used instead of walls to keep out the sun but still let in a little air. They can be hung from the tent structure with small clamps or bungie cords. Shade cloth also blocks 90% of uv rays, good for you and for your art!