If you're in an area with typically mild summers, WDW in August will feel very hot and humid. We went last August and it was actually 10-15 degrees cooler at WDW for us (i.e. we actually went to WDW in August to cool off!!! :lol

, but I could tell that a number of people were really feeling the heat.
There have been a number of good threads previously posted on things you can do to have a safe and enjoyable trip in the summer heat. So, you might want to try searching around for summer and August heat based threads.
EDIT- I think the one I was thinking of may be a little bit older, but while I'm posting...here are some summer suggestions (b/c I think it's fun and doable to go in August).
-plan mid day breaks when the heat is most intense- i.e. rest in a/c or go to the pool
-make sure to star touring the parks at park opening and take advantage of morning EMH- these will be the coolest hours. Late evening EMH can also be good for this.
-wear light clothes that wick sweat away
-stay really well hydrated
-wear sunscreen
-wear hats...it's amazing how the heat directly baking your head can zap energy
-wear walking sandals...cooler feet = cooler body
-carry a misting fan if you think you'll be really hot
-try and plan air conditioned attractions here and there to cool off- shopping in a/c and TS dining in a/c is also a good break from the heat
-minimize your load- if you have younger children...opt for a lighter stroller and lighter bag
-if you get any really hot days at home (or even a stretch with a lot of full sun) try and condition yourself by spending a lot of time outside
I'm sure there's more, but it's early and my brain isn't full awake
