We do August pretty often, next year will be with a 2.5 yr old in tow, but we stick to the basics.
Get out early in the a.m. to the parks, have a sit down indoor lunch at the park and then head back to the resort for pool/nap time. Basically, be out of the sun in the peak heat at 1-4.
We will typically head back to the parks at night around 5. Once the sun goes down, it's much better.
Often, we will walk through shops during the day as we go from place to place, because they are air conditioned. And remember that many of the rides have indoor queues so that helps.
We really like Frog Toggs and found that those helped to cool us down. We have the mister fans and use those as well.
And yes, LOTS of sunscreen!