Really, my best idea is air conditioning!!!! Anytime I feel too hot, I meander through an air conditioned shop, or plan to see a show or attraction that is air conditioned. At MK, the shops on Main Street are always ice cold, it seems. In Fantasyland, standing in line for Philharmagic gets you inside in an air conditioned waiting space. Hall of Presidents is also great for keeping cool, as is Country Bears and Pirates of the Caribbean. In Tomorrowland, Carousel of Progress is cool if it's open, and even the TTA can cool you down if it's hot - I always feel a nice breeze on it!!! If I'm REALLY hot, that nice area behind Tony's on Main Street always is cool and uncrowded - I think it's a photo op spot, but it's inside and air conditioned.
Animal Kingdom seems to be the "hottest" park to me, so I save it for another time if it's just brutal, or I go first thing and leave at lunch. MGM can also be hot, because shows like Beauty and the Beast and Indiana Jones, while fanned, still aren't air conditioned. I always plan a nice sit down lunch at both places - nothing like a big Coke at the Rain Forest Cafe after a morning of August temps in the Animal Kingdom!!!
Epcot's pretty easy - just pick a pavillion with several attractions and you can spend a few of the hottest hours without ever going outside. The World Showcase is better in the evening and again, the shops and movies are always air conditioned.
Fans and personal misters help a lot, and don't forget to drink that water!!! Which gets you the chance to visit many of the air conditioned restrooms!!!