Go back to the hotel in the afternoon!! I know this has been mentioned in passing, but I'd like to emphasize the importance of taking an afternoon break.
We go to the parks at opening. Before or after lunch, we return to the hotel for a nap and a swim and just to relax. Then we return to the parks in the evening. We've gone in the summer when the park hours are longer, so we were able to get everything done. We have small children who still nap, and by taking that break they weren't cranky later in the day and they could stay up until park closing. Mom and Dad get a break too, so everybody is happy!
Best of all, we avoid the hottest part of the day and usually the dreaded afternoon rain showers.