I am always shocked when I see riders show up for a 2 hour group ride with 1 bottle. Even in normal temps, that isn't enough. The general rule of thumb is a bottle an hour. When racing, make sure you show up to the event hydrated, and consider carry extra bottles in your jersey if it is hot. I have had some great results in hot road races simply from drinking a lot of fluids while my competitors did not. Also, remember the hydration is a long term deal - make sure you are drinking enough all the time, especially a day or 2 before a big race. If you are thirsty the morning of the event, it is too late.
Second, get used to riding in the heat. This is known as "heat acclimatization." The only way to do this is to ride in the warm and humid conditions. Like any sort of training, it works best to begin gradually and ramp up volume and intensity. If you tend to ride early in the morning, but have a race coming up in the afternoon, you need to make an effort to introduce progressively longer and harder rides when the temps are higher. You can get used to this fairly quickly, so working up to long rides in the heat can happen over a 2 week period.
Third, pay attention to clothing choices. Your body cools by sweating, so it is important to aid in the evaporative cooling process. Wicking base layers and venting jerseys go a long way to making rides enjoyable in the warm and humid weather.
Drink fluids, pick out good clothing, and get used to riding in the summer weather and your training and racing will stay strong until the fall!