Don't use thirst alone as a guide for when to drink. By the time you become thirsty, you may already be slightly dehydrated. To ward off dehydration, drink plenty of fluids and eat foods high in water content, such as fruits and vegetables. Increase your fluid intake during hot weather, especially if you work or exercise outdoors. Vomiting or diarrhea can increase the risk of dehydration, particularly in young children. Older adults who take medicine that increases urination are also at higher risk.