Last year I asked the same question, because it was my first time going in August. I was real nervous, but truthfully it wasn't that bad. In fact I am going back this August, so it couldn't have been too bad. Here is what I did and how the heat affected us.
1. Go to the parks early and leave mid-day. Unfortunately, you have to take the parks at a slower pace or else you will not make it. The sun is very hot mid-day. The mid-day pool breaks are great. Me and my family really enjoyed the pool at the resort.
2. Bring an umbrella, not for the rain, but for the shade. This helped me tremendously. I did notice a difference.
3. Stay hydrated. Drink lots of water even if you are not thirsty.
4. Take air condition breaks. Duck inside a gift shop to cool off and recharge.
5. Bring a washcloth from your room and wet it at the water fountains and wipe yourself off. I put my on my head. It felt nice.
The only park that I found I really couldn't do was Animal Kingdom. This park dosen't have as many cool gift shops and buildings to take breaks in. We had to leave early because I was getting dizzy and light headed. Other than that we survivied and are heading back this year.
Also as one post already said you can literally set your watch to the afternoon thunder storms. They don't last very long, but when we were there they occurred every day almost at the same time.
Don't worry about it and enjoy, after all you're at the happiest place on earth.