It certainly doesn't have to ruin your vacation! Our July trip was indeed horribly hot, but we just took it easy and used a lot of the tips that the other posters have suggsted. Keys for us were arriving early, tons of sunblock, staying super-hydrated, and using the mister fans. If we were getting really hot and the lines were long we'd duck into a cool place like Hall of Presidents, a store, or one of the neat indoor pavilions at Epcot. Ice Station Cool and the interactive fountains at Epcot are also fantastic for cooling off. We were always amazed by how "alive" we felt again after a short refresher in the air conditioning or a quick run through a fountain!
Also, we spent WAY too much money on cold treats like Mickey Mouse ice cream bars, but those treats are one of our sons' best WDW memories!
We usually spent afternoons resting in our cool dark hotel room or at the hotel pool; this recharged us for the evenings ahead.
Have fun!