My old show horse had this, and it is tough to manage. If at all possible, try to keep them inside during the day with good ventilation, especially fans. Some people will even put a mister over them. Make sure they do not have a thick hair coat - if your horse does, try to body clip them. Do not work them in the heat of the day - early mornings, late evenings will be your friends. Wet them down before you work them, during, and afterwards. You may have to stand them under a hose for 10, 15+ minutes to cool them down.
A supplement that I used was called ProSweat. It's not perfect, it won't make them normal, but it helps. There is another medicine, but I can't think of the name and for my horse, when I had to use it, it was around $100 a week.
The most important thing is to use common sense. If they start overheating, get them cooled off. Pay attention to them. They take extra work, but can be managed. Let me know if you have any other questions, and I'll try to help.