I believe I've mentioned before that I enjoy bicycling; however, in central Georgia, doing any kind of exercise outside in the middle of the summer is extremely challenging. Before I start my bike rides, I make sure I've hydrated myself, and I make sure I take plenty of fluids. Usually I mix a nutritional fruit drink with water.
Drinking large volumes of water is a must for cycling. Nearly every calorie of heat you produce must be dissipated by evaporating water from your skin, which adds up to tremendous volumes for replacement: one large water bottle of 750 mL or 24 oz per hour of riding.
By the time your feel thirsty, you are already dehydrated! Drink water preemptively. Drink more water than you think you need, and you will simply pee out whatever you don't need. A good measure is that if you are not generating a relatively clear stream of urine every two or three hours, then drink more!