This tip is kind of psychological: make the heat/humidity part of the experience. I know that sounds weird. The first time I visited Florida, I felt practically choked by humidity when the sliding doors opened at MCO. Arriving on property, I saw a supervisor wearing a suit in the wet 95 degree air, and the poor cast members in polyester coats at the Haunted Mansion, and wondered why the heck anyone would choose to build a resort in this place.
Once you accept that it's going to be hot, and you're going to be sticky and sweaty and covered in sunscreen goop all day, it's kind of liberating. In fact, at the Animal Kingdom, it even felt like it was part of the theming. Like I had really been dropped into some far off tropical safari, like in those old movies with explorers arriving in some colonial outpost.
I secretly missed the heat last time I went there in the winter.
Also.... buy one of these water bottles with the double insulated walls. Yes, you can get free cups of water at the counters, but I've found that having these encourages me to hydrate more often all day, and you can carry them on rides. It keeps ice chilled for many hours. My favorite is the "Polar Bottle" (
link) because it's lightweight and you can throw it in a sackpack. At the resort, I fill it up all the way to the top with ice and water, and then refill it throughout the day. It might need new ice throughout the day. Remember: if you have to go to the restroom frequently, despite the sweating, you're doing a good job at hydration.
Taking a break is a good idea. There's so much to do at WDW, it's better to take your time and make it a relaxing vacation than to go full throttle all day every day.