It can be very difficult Sharon, not just because of the heat but the crowds too.
We are going twice this year including in August, it's not an ideal time at all but because of the school holidays really.
Personally I would not advise any first timer to go July/August, mainly because there is always the tendency to try and do too much - for us it's not too bad because we have been so many times now we can just pace ourselves. However, if you try and rush round and pack so much into a couple of weeks it will be exhausting.
You need to have plenty of rest between your park visits and to relax as much as you can.
Getting to the parks early and taking a break at lunch time is essential, you can always return in the evening, this way you should avoid the hottest times of the day and in some cases the busiest also.
Drink plenty of water, wear loose comfortable clothing, do not rush around too much..just the general advice really.
Be flexible in your plans so that you can have days of rest, Florida isn't all just about the Theme Parks.