For many years, the only time we could go to WDW was in the summer so I know plenty of ways to handle the heat.
As hard as it may be get to the parks early in the morning do it and then go back to the resort around 12-1 pm. Not only do you get out of the heat and sun, you get a break from the crowds. Use Extra Magic Hours at night once the sun has gone down. these 2 things can get you on a lot of rides in the shortest amount of time.
If you do stay in parks all day.. make your big meal of the day lunch.. but make it a later time after 1 or 2 is even better. This will get you out of the most intense heat/sun of the day. Plus, lunch is cheaper than dinner. For us, we usually have breakfast in the room, our big meal is lunch and then something small for a snack/light dinner around 7 or so. I know that may be late for some.. but it is vacation and schedules might be off anyway
If at all possible, take a day away from the parks. For example, if you go to the parks Monday and Tuesday, take Wednesday off to enjoy the resort, pool, etc. Again a more low key day away from crowds... it will seem cooler!
Stay hydrated and get into the shade if possible. Be careful to just rush into buildings. My body doesn't like to go from really hot to really cold (other than Test Track

) I have to gradually move from the warm into cold. So, before I go into a building I will stand in shade for a bit before moving in.
Take breaks. Try to find attractions that can keep you inside and that are longer.... spaceship earth, TTA, Railroad, great movie ride, shows, conservation station, etc.