I lived in South Florida for many years, so I can tell you all about the heat! The heat in July is a killer, but the humidity is your biggest challenge. The air is thick and wet, almost like trying to breath through a washcloth. Get used to not being completely dry, -- you will feel damp and sticky for most of the day. Most importantly the humidity can zap all of your strength.
As most people have already said, make sure you drink a lot of cold water and bring a spray bottle with a fan so you can mist yourself. Get cups of ice wherever you can, drinking ice cold water or chewing on ice will help you stay cooler longer. I would stay away from soft drinks and alcohol, at least while you're walking around the parks, since they can dehydrate you. Make sure to note on your park map where the mist/cooling stations are in each park. You will want to visit these often.
Also, be careful what you eat. Try to eat light and stay away from fried foods and heavy meals, they will just make you feel heavy and drag you down. I know it's hard to eat light at Disney, but I promise the lighter you eat, the better you will feel. You may want to wait until later in the day or before you go back to the hotel to have a big meal.
Remember to use lots of sunscreen and wear a hat that protects your scalp and face from the sun. The Florida sun can burn you quick! Also wear light clothing that breathes, stay away from nylon, spandex, etc. Anything that wicks away moisture is your best friend.
Also, Florida isn't like many other places where it cools off in the evening. Even if the temperature drops, it won't drop enough to make you comfortable, so prepare for the evenings the same way you prepare for your day in the park.
Thankfully, virtually every building is very air conditioned. Whenever you feel the need, go inside, sit down, and cool off! Heat stroke is always a possibility in summer, so if you feel weak or faint, make sure to seek medical attention. The first aid clinics at Disney know exactly what to do to help you.
I know this sounds like a lot, but if you prepare in advance, you'll be fine.