I hate heat, but it's August or not at all for us and we make it work. Drink plenty, seek shade, wear your sunscreen and a hat, take time out in the AC and make of note of which rides give you a nice break in the cool (especially if they have seats as well!) so you know what to do when people overheat (we rode Ellen's Energy Ride for the first time this summer because it was hotter than the surface of the sun that day in Epcot, and it made a great break!)
We find that we need to plan to be in the parks for 2 parts of each day. We go for two weeks, so RD to closing every day isn't sustainable for us. We would do RD and afternoon, then head back to the resort (if we were still up for more, we sometimes headed back for an evening in the parks, but we went with what felt right), or go midday/later afternoon sometimes and stay through until the park closes. We made sure we planned plenty of down time (that pool at CSR was awesome!) and then added park time in where we could handle it. We found hopping parks sometimes gave us the break we needed too to give us an energy boost.