Here are my tips, as someone who's been to WDW twice for week-long visits in mid-to-late August (including this year, with "real feels" of 115 degrees in the middle of the day):
Go to the parks early, leave at lunchtime, take a refreshing siesta/shower/swim back at your hotel, and come back around dinnertime, when temperatures start to go down.
Pack lightweight, moisture-wicking clothing, in styles and fabrics designed for athletic pursuits or sports (e.g., golf, hiking, tennis). It dries quickly, and tends to pack like a dream since it's all thin and doesn't wrinkle. In the same vein, wear comfortable, well-broken in shoes, with at least one pair of quick-dry hiking sandals or mesh sneakers to put on if it's expected to rain. Bring extra shirts and socks, so you can change into fresh ones after your mid-day break.
Plan your day so you're visiting rides with outdoor queues early in the morning, or late at night. The rest of the day, you should just be hopping from one air-conditioned space to the next. Include frequent rest breaks, and stop at CS locations for free iced water, throughout the day. (We prefer not to carry heavy water bottles around with us in the heat, but that's an option, too.) Hydration powders that add electrolytes and flavoring (sold at most grocery stores) are a great option to improve the taste, if you're not a fan of Florida tap water.
Pack light when you head out to the parks: your park bag should be small, breathable, lightweight, and able to be worn under your rain poncho if needed.
Use cooling towels and/or fans, whatever works for you. Don't forget sunblock, hats, UV-blocking swim covers, and sunglasses.
In case of a downpour or storm, bring breathable lightweight rain ponchos (we like Frog Toggs brand, but there are many on the market) instead of cheap plastic disposables (which are too flimsy if it's windy, and can be very uncomfortable in the heat).
Don't be intimidated if the heat hits you like a ton of bricks your first day or two. You body will take a little time to adjust. You're still going to feel hot: but it will get less bothersome.