Ever since I was little I have had problems with the heat. In addition to tips from the others (I also drink 1-2 bottles of water per hour and use a neck scarf), I have a few that work for me:
-The spray bottles are great (buy one at
WalMart before you leave and save yourself about $10). To give them a little extra "punch," fill it with a combo of ice
and a little water. Fill it up before you leave the hotel. As you enter the park and the heat really hits you, you'll have ice cold water to spray yourself with. Also, ask for a cup of ice at restaurants and fill it up throughout the day. I explain to the CMs why I need it and they think it's a great idea!
-I alternate 1 indoor A/C attraction with 1-2 (maybe 3 if they're close like in Fantasyland) "regular" attractions. That way I'm getting my body into A/C fairly often. For example, I'll do Jungle Cruise and Magic Carpets and head straight for the Tiki Birds or POTC (I consider it A/C, since almost the whole queue and the ride are inside). Or, Small World, Snow White, and Peter Pan (if the line's short or if I have a FP), then Philharmagic.
This will work for just about any land in MK, and it's pretty easy to do in Epcot and AK, and especially MGM.
-One last tip. I have found over the years that I cannot do World Showcase in the afternoon. If you get to Epcot when the park opens, it always seems to end up that you're doing WS at like 3 or 4, and it's just awful. I usually do Future World in the morning, go back to the hotel and swim or do DTD (or even another park) then head back to do Future World at night.
Hope these help!