I would avoid the middle of the day and make sure that when you take a mid-day break that you have them shower or swim and get totally cooled off.
Use the mister fans,
Get wet in the water play areas
Tour the parks with some thoughts about the heat. Ride a few rides, then take in a show and cool off.
Every time we see a show, we get something COLD to eat or drink.
Return in the evening as late as you can.
Don't over-do it. That is probably a bigger issue.
This isn't really a heat issue exactly but I found that I really liked having our rental car to get in at the end of a long, hot day. We could relax as soon as we got in the car and not after the added 20-30 minutes on the bus. We also kept a cooler in the car with capri suns and water. They were cool at the end of the day, not cold.
Some days we did the bus, as DH had the car for a work conference. I will never visit WDW without a car again. Maybe it is because we are not used to public transportation around here, but I really liked having the car.