The Best defence against dehydration is a good offence -Drink Often!- at least 1 litre per hr for moderate activity in moderate conditions. It’s also better to drink continuously – 6-8 oz every 15-20 minutes is recommended by the American Collage of Sports Medicine. Remember that thirst is a delayed response, and by the time you’re thirsty, your body is already dehydrated. So pre-hydrating is vital before taking part in any summer exercise. The ACSM also recommends drinking between 14-20 oz of fluids two hours before exercising.