Taking steps towards preventing dehydration is a smart move if you want to exercise to the best of your abilities.
When you are working towards preventing dehydration, you want to take into consideration the atmosphere you are exercising in as well. If you are in a very hot or humid climate, you'll find that your going to lose body fluid at a much faster rate and therefore, will need to be particularly careful to ensure you are taking in enough.
The other factor to consider when preventing dehydration is the intensity you are working at. More intense exercise will speed up the rate of perspiration, further causing you to lose water. Additionally, since performance decreases tend to be more obvious when working at these intensities, any degree of disruption of water balance in the body is bound to become more noticeable.
The best way to avoid this condition from happening is to try and consume eight to ten glasses of water during the day, to ensure that you are not starting at a deficit, and then to also drink a small amount of fluid every 15 to 20 minutes while you are exercising. Some may want to also consume one cup of beverage before beginning, if there are no issues with cramping, and most should consume another cup after the exercise has finished.
Note, however, that if you are exercising for an extended period of time you'll want to use an electrolyte replacement beverage rather than just pure water to make sure the sensitive balance of potassium and sodium does not become disrupted.