Staying at an on-property Disney hotel is a good start. You can commute to the parks in air-conditioned comfort using Disney bus, monorail or water transportation. This is definitely a better choice than driving a car that has been baking in the sun for hours.
Another incentive for staying on a Disney property is that you can take advantage of the Extra Magic Hour. On designated days, a Walt Disney World theme park will open an hour early for Disney Resort guests only. With that extra hour, you have time to take a break during the afternoon for a swim or a nap. Some resorts even have water activities, such as water skiing and surfing lessons, which are perfect for a hot day.