We've done 2.5 week long trips each of the past 4 summers, and have made probably 8 or 9 summer trips in total. How you handle/deal with it has a lot to do with how you handle heat in general and how acclimated to it you are.
Personally, I don't follow the theory of needing a mid-day break. If you want it, there's nothing wrong with doing it. However, even on the hottest of days, it's not necessary IMO. There are countless attractions that are indoors and/or have lines that are in a/c. You can spend the entire afternoon in a/c, except for the time you have to walk from one attraction to the next. Things like FoLK, CoP, Philharmagic, Dinosaur, Muppets, Storytime w/Belle, Laugh Floor, Soarin', Living w/Land are all perfect mid-day attractions. So is Kali if you want to get wet and cool off. So for us, instead of wasting time travelling back and forth to our condo (we stay offsite, but it's still the same concept if you stay onsite), we stay in the parks until we're done then leave. If we want to swim, we do it when we get back to the condo...or we'll swim first and then go to the parks mid-afternoon and stay through the evening.
Drink plenty of fluids. You can get free, filtered ice water at any QS place.
Light clothing. Doesn't have to be moisture wicking (not that it's bad, it's just not necessary), cotton works well.
Spend time in stores.
Do the outdoor rides early in the morning or late at night...Astro Orbiter, Tomorrowland Speedway, Safari, PW, Teacups, etc...
Some people like misting fans or
cooling towels. Personally, I find them gross and they only make things worse.
We love, love, love summer at WDW. A hot WDW is light years better than a cold WDW for us. We've done cold days at Disney and they were miserable. Just use a bit of common sense and you'll do fine.