Do you acclimatize? That is, do you start riding when it's cooler so your body is gradually exposed to higher and higher temperatures? Or, do you start riding when it's already "warm?" To do the latter can be a real mother. A 90 degree day is very tolerable if you've been riding regularly from when the temperature was only 70-75 degrees. And what are you wearing? Some of the "high-tech" clothing now available can make a big difference. Also, you are wearing lighter colored jerseys, right? Of course you are.

Another trick: place one of your water bottles in the freezer overnight. Carry that fully frozen bottle of water along with a regular bottle of water. Those sips of 35 degree water you get as the ice melts will cool your body's core temperature. I suggest doing this with the new insulated bottles now available. If you start with one fully frozen, you will likely have cold water to drink for at least the first hour on the bike, maybe longer. And should it remain frozen too long, freeze a bottle that is not entirely full. Then add some of the warmer water from your other water bottle to melt the ice as you ride.

These things, and what others have said about remaining as hydrated as possible, will make a huge difference. Just remember you actually hydrate when you're not riding. You can only replace but so much water when you on the bike. First, it's not there. Second, your stomach probably can't hold the equivalent amount of water you're losing through sweat.