The planes really don't care about the temps, but make sure you have some shade. Clear canopies can raise the temps inside the plane, and you don't want to overheat recievers and batteries. You may notice some wrinkling of the covering if its cheaper covering or not properly adhered. The engine will probably have to run a little leaner, as the air isn't as dense, and you're going to have less power. If you have an asphalt pit area, keep the planes off the asphalt until you're ready to take off and fly. Keep your fuel in a shady spot, and keep the bottle capped to minimize evaporation.

Most importantly, take plenty of water with you, and drink it constantly (I take 1/2 gallon jug with me). Its very easy to get distracted and then get dehydrated. Stay away from caffienated beverages.

Brad