Your body is made up of about 75% water. By the time you feel thirsty, your body has probably lost 500ml or more of water.
Symptoms of dehydration can include headaches, a sudden episode of visual snow, dizziness or fainting when standing up.
Dehydration is best avoided by drinking plenty of water. The greater the amount of water lost through perspiration, the more water must be consumed to replace it and avoid dehydration. Since the body cannot tolerate large deficits or excesses in total body water, consumption of water must be roughly concurrent with the loss (in other words, if one is perspiring, one should also be drinking water frequently). Drinking water beyond the needs of the body entails little risk when done in moderation, since the kidneys will efficiently remove any excess water through the urine with a large margin of safety.