Dress light, don't wear cotton.
As Brad said, I also like to carry some towels around. Usually I just keep one damp towel to wipe off sweat with. If it's a tournament, I actually carry three. One damp one to wipe off any sweat/dirt off the disc, one dry one to dry off the disc, and another damp one to wipe my hands and face with (don't want that same sweat going on discs later). If it's a really hot day I will dip a towel in cold water and drape it over my head. I guess towels are just important for my game on the really hot days as they are on rainy days.
As always, don't forget to stay hydrated. I go through about 1 or 2 32oz Gatorades per 18 holes on really hot days.
Last but not least, try to find shade where you can. No reason to stand out baking in the sun while waiting for someone in your group to make their shot. Even if it means going out of your way a bit, it helps in the long run.