Do you have an event coming up where you know the mercury will be reaching the triple digits? Full heat accilmation can take weeks, but you'll see some benefit in as little as a few days. If you have an event that is going to be in a warmer climate that your normal training rides, you can do some workouts in the heat the week before to get yourself ready. Increased blood plasma volume and a shift in the composition of your sweat are some of the changes that will begin to occur. If its not hot enough where you live and train, just do some trainer sessions and get that core temperature up. A big pool of sweat underneath the trainer is a sign that you are doing it right. Just be sure to hydrateduring and after the ride!