Keep a water bottle on you. the Brita filter ones work great. You don't want to get dehydrated.
Sunblock
Dress light and in light colors
Duck into shops, rides, shows that are AC to get cooled off for a bit; this really helps and is easy to do everywhere except AK. AK is probably the hottest park.
Most of the parks have places where you can walk through a mist thing; Epcot has that Igloo thing that is very cold.
Carry a spray mist personal fan.
Again, I think at most of the parks, just getting somewhere where you can sit in the AC indoors is the biggest help; but also drinking lots of water.
Have a great trip!!!