We visit the world every year in August. The heat ranges from miserable to brutally miserable lol.
Stay hydrated. Take advantage of the free ice water from QS locations. If you can't tolerate the taste, bring along flavoring like mio.
A wet wash cloth / hand towel can be wonderful when the heat really starts to get to you. Personally, I am not above putting one on top of my head and walking around that way.
Pay attention to what your body is telling you. When you need to sit down for a while and take a break, do so. That is a good time to have a drink, maybe a snack too. Cool off a bit, rest a bit, recharge a bit.
Lunch should be someplace with indoor seating if at all possible. An hour sitting down in the air conditioning can do wonders for you.
We almost always schedule a table service meal for supper, expecting that it will take an hour and a half. This, again, is a fantastic break from the heat, and an opportunity to recharge a bit before the evening.
Long shows with short lines, and shopping are some of our favorite afternoon activities. We try to tour hard in the morning, hitting as many rides as we can before the crowds, and heat build, then a leisurely lunch, and an easy afternoon with plenty of indoor shows, and shopping. Follow that with a nice long TS supper, and we are ready for the evening.