We went last July and we had a fabulous, FABULOUS time by using the tips that folks posted above. The water bottles with fans attached are absolutely key. They helped us so much. If at all possible, buy them before you go. I know they have them at Bed, Bath and Beyond, and other large stores with a home goods section. One brand name is Squeeze Breeze. They are about $8. If you go in summer, you will see them all over the parks, but there, they were $17 last summer. At the resorts, they were $16. Don't leave home without them!
Also, remember that there is so much in WDW that is indoor and air-conditioned. At MK, you can cool down very nicely at Mickey's Philharmagic, for example. Peter Pan, Pooh, and IASW are also inside. At PoC, it's an inside ride, and the queue is also inside. There's Hall of Presidents, there's seeing Woody and Jessie in Frontierland, there are just all kinds of places to go in and cool down. Epcot has even more, and MGM has plenty as well. AK has fewer, but still has Tough to Be a Bug and FotLK, among others.
The other great thing about summer is there is still so much park time left after the sun goes down. I found evenings to be fantastic. The whole key to summer is to be prepared, and not to try to go commando. Enjoy, relax, you're on vacation. You will be amazed how doable summer is when you have a good plan and you stick to it. I sure wish I was in your shoes and in the position to go again this summer. Hopefully 2007.