I don't really fit in with the crowd here, but I'm an 18 year old who's battled the Disney heat in August for the past 6 or 7 years, and before that the July heat since I was 2. We're from NJ and go every summer, and I'm really heat sensitive, getting dehydrated at home quicker than you can imagine. So we've found a lot of tricks over the years-
-always have a bottle of water, cold or not, because it just takes even a few sips of some water to get rehydrated enough to go find cold drinks
-keep sunglasses off and hats on, cause stuff touching your face in 90 degree heat isn't fun at all
-get maps before you go and circle all the places that have spare room to cool down, like in Electric Umbrella in EPCOT, around the side there are bathrooms and benchs and it's always air conditioned in this big hallway
-if you, or anyone, starts to feel dizzy or stops sweating when you should be, or get a little pale, find a shaded curb or bench and sit with your eyes closed. can't tell you how many times i've done this. it always gives me that push to get where i can really sit and relax.
-don't spray water on yourself too much, because you're setting yourself up for the water to evaporate and hotter skin
-the washcloth works, I promise, but make sure it's not too cold because once you put it on, you'll never take it off and when you do, the heat feels even worse
-when you get off of a ride, take a sip of water before you step into the sun, but don't swallow it until you're out in the sun. maybe just a mind trick but having that gulp for the second you hit the sun makes it seem less hot.