I've been to WDW twice in August, and it's great! Heed the advice about going early to the parks (especially the water parks), and I agree that the evenings are probably the busiest times. I would absolutely recommend doing E-ride night (if they are still offering it-not sure). The lines in general were not bad at all. We only waited longer than 20 minutes once (Rock N' Roller Coaster), and that was because it was mid-afternoon. I was all set to do the 'get there early, leave at noon, come back later' thing, but my DD wanted to 'go, go, go!' So, we kept on going. Eat lunch early as well (11:00) and you'll get the double bonus of smaller lines due to lunch and noon-naps.
Absolutely drink lots and lots of water. All the time. Drink when you are not thirsty, because if you wait until you're thirsty, it's too late. One thing that worked for me, was making sure we took a little drink at least every half-hour, more if needed, but not less than every half-hour. My daughter and I never got sick, or even too tired, and we didn't take any more bathroom breaks than usual. The Brita water bottle was the best money I ever spent for a vacation item. I got mine at Wal-Mart, but I'm sure they're everywhere. On the hottest day (97-98 degrees, humid of course!), I kept refilling the bottle with ice chips from the vendors. They weren't happy, but I was hydrated! And the water was always cold!
I've also traveled in March (pretty good) and April (way too crowded-won't do that again), and am probably traveling this year in October, so I'm concerned about the previous post mentioning horrible crowds! I'd go in August instead, but my cousin is getting married in October (he's a CM-lucky guy!), so that's when we're going.
Have a great time!
-Sarah