we were there last June, the last week of the month for our first ever trip. It was very hot, but we planned for that and make sure we had lots of sunscreen and waterbottles.
The crowds were killer some of the days. I don't know that I would go back again in the summer unless I had no other option.
We found it very helpful to:
Get to the parks early. We were there about 1/2 hour or so before opening each day. There were still lots of people already there, but that allowed us to get on what we wanted first thing, then take it easy and rely on fast passes later on as the crowds grew. The crowds didn't get astronimical until around 10 or so most days. (Which coincidentally is when it started to get substantially hotter as well!
avoid the parks with early morning hours. This tip came from the unofficial guide. The park with early hours is typically going to be the most busy for that day, We also avoided late hours as well.
Resign yourself to the fact that even if you've been able to do it all before, you probably won't with the sheer volume of others trying to do it all too. Make up your "must-do" list, your ""would be nice to do" list, and your "not this time" list ahead of time then stick to it while still being flexible to those oh-so-wonderful spontaneously delicious Disney moments as much as you can, and everyone will be a little happier.
Stay hydrated!!!!
Expect rudeness, and be kind in return. You'll feel better about yourself, allowing less stress to build, and you never know how your kindness may change how someone else reacts later to someone else.
Don't "sweat" (ha ha!) the small stuff.
Enjoy!