We go last week of August and it was REALLY hot this year on a few of the days.
BEST thing I every bought - who knew??? - was a silk fan in China (Epcot). Great! I used the misting fan last year and I liked that, but the silk fan was way more convenient to carry around. And everytime I used it during line waits or whatever, someone would ask me where I got it! LOL! (Now it is in my draw at school - my official hot flash fan).
My other tip is to walk thru the stores when at the parks. In MGM, for instance, you can walk down the street from ToT toward the exit, strolling thru the connected air conditioned stores. You can do this in MK, too! I find when I feel overheated, just a minute or two of cool air helps.
As others have said, don't zip around in the heat. Enjoy!