I have been to WDW in mid-late August the past two years in a row, and we are going again this year. Personally, if I had a choice, I would never go during this time of year, but I'm a teacher. We prefer the lower crowds and heat to very high holiday crowds (my only breaks would be Thanksgiving, Christmas, Easter, and Memorial Day) and cooler weather.
I think all above posters have very good advice. Personally, my DH and I find our Frog Toggs
cooling towels to be a
godsend (they don't look pretty but they WORK), and a misting water bottle would be great, too. Get ice to put in the water bottle at every possible opportunity. Don't bother putting on makeup or dressing up - you're going to be sweating within 5 minutes of walking outside. We just wear athletic clothes every day. Make your ADRs during times when the heat is the worst (noon to 5). I personally don't go in the shops (we don't do souvenirs) to cool down. Instead, we plan to go to rides that have air-conditioned queues and shows when the heat is the worst.
Here's a list of attractions where you can find relief. I tend to over-plan every single trip, so this is what I have learned! I did not include water rides.
Magic Kingdom
Space Mountain (I don't mind waiting for long periods of time there)
Buzz Lightyear's Space Ranger Spin
Philharmagic
Carousel of Progress
Pirates of the Caribbean (should cool you down once you're on the ride!)
PeopleMover (great breeze!)
Country Bear Jamboree
Hall of Presidents
Enchanted Tiki Room
Hollywood Studios
Great Movie Ride
Star Tours
Rock N Roller Coaster (partially)
Tower of Terror (partially)
Voyage of the Little Mermaid
Star Wars Launch Bay
One Man's Dream
Epcot
Soarin'
Livin' with the Land
Mission Space
Test Track (partially)
Ellen's Energy Adventure (great for a snooze)
Impressions de France
The American Adventure show
Voices of Liberty (inside the American pavilion)
Mitsukoshi store at Japan (I know this is not exactly a ride or show but my DH and I spend at least a half hour there every trip because it's huge and engaging)
Mexico ride
The show at China and Canada are air-conditioned, but you have to stand. I don't find them relaxing because of this.
The Seas with Nemo
Animal Kingdom
Dinosaur
It's Tough to Be a Bug (partially)
Festival of the Lion King
Finding Nemo show
One thing no one has posted is that you can relax in the lounge of any nearby hotel during those hot times if you don't want to go back to your hotel. We found a lounge at the Swan that was incredibly cool and comfortable. At MK, you could relax at any of the monorail resorts (who doesn't love riding the monorails, anyway?). At Epcot (walking) or HS (take the boat), you could go to the Yacht & Beach Club, the Swan & Dolphin, or the Boardwalk.
I do want to mention something that no one brings up. Everyone says it's a great idea to go back to your hotel mid-day, but it can take a while to travel back and forth. For my DH and I, we oftentimes found ourselves so heat-exhausted that all we wanted to do when we came back was just to say in our hotel room in the AC. We would get back and relax and feel so lazy that it was hard to get ourselves moving again and motivated to go back out into the heat. I've never seen anyone else say that, so I don't know, maybe that's just us.
I don't want to scare you off for that time of the year, so I hope that this post didn't do that! Any day at Disney is better than no day at Disney. These are just some things I've learned over the past couple of years and wanted to pass on. Like I said, though... if you can really go anytime during the year, this is really not the best time to go, weather-wise.