Another good heat related tip from professional triathlete Doug MacLean:
A little more about racing in the heat… Many athletes from cold weather areas are traveling south to do some May racing in such locations as Florida, Texas, and the Caribbean. The heat of those places can be quite a shock to “un-acclimated” northern athletes.
In order to prepare, it’s helpful to do 2-3 “heat workouts” per week, where you put on 1-2 extra layers of clothing for the workout. Or, if you’re cycling indoors, you could simply choose to not use a fan, and wear a long sleeve shirt or sweatshirt for the entire ride. But remember to bring extra fluids on these sessions, because you’ll be sweating quite a bit more!