Water, water, and more water. If you ride with a group, take a sip of water each time anyone else takes a drink. I try to take a sip every few minutes. I personaly drink just under 50oz of water on my daily ride (22miles).

Another thing you should never go without, is sunscreen. It is proven that sunscreen can reduce skin temp by 20 degreesF! I use an SPF 15 sunscreen on all my summer rides.