Let me second - or is that third - the advice given by tink 2257 and SelectTheGold. We'll be there May 31 - June 6, and we've been to WDW in the hot & busy summer times before.
1. Get up - and get to a park - early; preferably at rope drop, at a park with EMH-AM if possible. It is still quite pleasant that early in the morning, and you will get so much touring done in first 2 or 3 hours.
2. As the park starts to fill up around 11AM or noon, head back to your hotel. Rest, cool off in the pool, put your feet up, take a nap - whatever you need to do to recharge. This is the busiest - and hottest - time of day at the parks.
3. The parks are open later this time of year, so take advantage of it. Head back to a park at 4 - 7 PM (depending on park and hours) - you'll see hundreds of people leaving as you re-enter. Plus, we just LOVE the parks at night, all lights and shadows. EMH-PM is a mixed bag: much busier than EMH-AM, but if the extra 3 hours means a park is open until midnight or later, this is prime park time.
IMHO, the worst thing you can do (and we have friends who have done it) is to sleep in, go out for breakfast and show up at the MK at 11:30. Lines will be long, the sun will be hot, and tempers will start to flare.
IMHO - YMMV - Be well!