The past few times I've gone during the summer, I tend to go on a Saturday lately -- since there are blackout dates, Saturdays seem less crowded than Mon-Wed in the summers the past few years. LOTS of water -- I carry a bottle with me and refill at water fountains and drink it down in lines. Shady spots - Tom Sawyer's Island (or whatever they call it now), the area back behind the Winnie the Pooh ride can be nice and quiet, also the area underneath Hungry Bear has shade and chairs for a quick break.
Tiki Room's great for an air conditioned break, so are the cartoons on Main Street. Riding the train for a full loop is nice as well, just to relax and enjoy people watching. Have a great trip! :)