We've lived in Fl for a little over a year, and we prefer to do Disney in the cooler mos. This year we HAVE to go in July because of dh's sea sched. Yes, he couldn't be gone for 6 mos. without a good WDW send-off. lol! All the things mentioned are good, especially splitting up the day with a break for swimming or just relaxing at the hotel so you can gear up for a late night in the parks. We used to be theme park commandos, though, and to beat the heat and try to go all day we'd have a sit-down lunch as a break. If you don't want to leave for a break, then a little planning will make a big difference in beating the heat. Plan to be in certain attractions with A.C. and seating during the heat of the day, 12-4. That's the perfect time to go to:
MK -
Hall of Presidents, Haunted Mansion, Pirates, Tiki Birds, Mickey's Philharmagic, Carousel of Progress and to shop Main Street. Yes, many rides in MK involve a.c., but we like specific ones in the heat of the day for the dark, relaxing feel. Mid day is the best time for Splash Mtn, but we rarely do that one.
Epcot -
We head to Innoventions (can spend hours in there), The Living Seas, or Spaceship Earth. If you're in WS, then American Adventure, Norway, and Mexico give you a seated break. FW is better for an afternoon break, though.
MGM-
We go to Voyage of the Little Mermaid, Millionaire, The Great Movie Ride, Muppets 3-D, Magic of Disney Animation, and the Walt Disney Story (might have butchered the name of that one). My kids can do laps at the animation class. They kept going last time trying to draw every character that was taught.
AK -
Flights of Wonder (no a.c., but shade & benches), Rafiki's Planet Watch, Dinosaur, and Kali River. There's also Festival of the Lion King, but it's crowded, and benches have no backs, so it's not restful to us.