We went in August last year and found the following:
Don't drink just water as you need sugar because of the heat, I do not drink coke etc as I am diabetic but we found that we needed the sugar badly and drinking fizzy, sugary drinks helped immensely. with all the sweating you do your body needs to be compensated and sugary drinks really help.
Take advantage of early morning entrances - we used to be there for 8am and leave the parks by 12 noon as it was just too hot
The best thing that Disney ever did was late park opening hours, these were a god send as it enables you to go back to the hotel and rest during the hotest hours. We entered Magic Kingdom one evening at 7pm and left at 2am and it was brilliant
You should wear a hat if swimming as the sun really beats down and reflects off the water. People tend to huddle in shaded areas of all pools as even in the water the sun attacks you and you can boil!
We always booked dinners for around 5pm as it's still hot then and can give you a much needed break.
Always look at the weather forcast and if it's going to be just too hot for you go to Disney Quest - we did this and it was a great relief
Basically we found an August holiday is taken at a much slower pace than other months as it's just too hot. Once we had learned to slow down we started to enjoy it much more and still had an absolutely brilliant time. We made sure that we were in our room most afternoons when the storms came, we had some billiant thunder storms and thoroughly enjoyed them.
Hope you have a brilliant time.