well, I only don't ride if its over 100. That being said, my horse in excellent shape for competetion and used to the heat.
To keep ME cool:
-get a helmet with vents, makes all the difference
-we keep one of the sports coolers in our indoor, next to some paper cups.
-when packing to go to the barn, bring multiple water bottles....i usually bring a bottle of gatorade and two or three water.
To keep my horse cool:
-keeping a bucket of water in the ring for walk breaks(sips only)
-picking a trail in the shade with creek crossings along the way
-using thin saddle pads
-doesnt hurt to jump off and sponge some cool water on their chests.
When riding in the heat, take extra good care of you and your horses after a ride. It usually takes me about 45 minutes to safely hydrate, cool, and make sure he is comfertable to turn back out.