We visited WDW in August last year, July the year before. The most important tip for surviving heat is STAY HYDRATED! Instead of buying water, we simply asked for water at any and every counter service place or drinks stop we encountered.
Look for cool spots to relax in periodically (air conditioned attractions, stores etc).
I've never tried the mister fans, but a washcloth or swimmer's towel (chamois) soaked in cool water and applied to neck, face etc kept me cool. If you're really hot, and you don't mind forgoing your look, soak your head if you're overheating. Personally, I decided early that my vacation was likely to be 12 consecutive bad hair days, so I rocked a hat or a pony tail most of the time and didn't care too much about picture perfect hair! The hat was a must for sun protection anyway!
Wear moisture wicking clothing that will dry quickly. Don't forget to apply sunscreen often - not a "cool" tip, but definitely a necessity in the August heat.