Rule #1: Hydrate properly BEFORE you begin exercise and slowly maintain hydration otherwise you may never catch up. If you have any doubt, monitor your urine color. If it's school-bus yellow or darker you are under hydrated. Try starting strenuous exercise when it's almost clear.
Rule #2: Make sure you replace electrolytes and carbs. That can be mixed in with water or nibbled as a replacement bar or gel snack. Note that "energy" drinks and bars are not good for hydration. The Air Force has recently issued a health warning about those: Energy drinks in downrange DFACs
The best details on the rest of the rules can be found at the Department of Defense's Human Performance Resource center: Staying hydrated during exercise.
Here's a taste of that well-written article:
"Water and electrolytes (sodium/Na+, potassium/K+, chloride/Cl–, and others) serve very important roles in the functioning of the body, and sweating can lead to excessive losses of both critical nutrients if not properly replaced.
"Dehydration and electrolyte imbalances can adversely impact health and exercise performance. The magnitude of fluid and sweat losses during exercise depends on the intensity of the exercise, environmental conditions, and the type of clothing worn during the exercise.
"To avoid excessive fluid and electrolyte losses, a person should begin exercising in a well-hydrated state. About two hours prior to strenuous exercise, drink approximately 20 fl oz (500ml) of liquid to ensure proper hydration at the onset of exercise.
"Consuming sufficient fluids during exercise will influence cardiovascular, thermoregulatory, and muscle function, as well as hydration status.
"To avoid dehydration, 13–32 fl oz (400–1,000 ml) of fluid should be consumed every hour by drinking small amounts frequently: 3–8 fl oz (100–250 ml) every 15 minutes or 8–11 fl oz (250 – 330 ml) every 20 minutes.
"Water is fine if the exercise is of short duration, but if the exercise is longer than one hour, the fluid should contain carbohydrates (from sugars) and electrolytes (from salts).
"The addition of carbohydrates to a fluid replacement drink can enhance intestinal absorption of water and help maintain blood glucose concentration during exercise, which may preserve muscle glycogen (sugar storage) and thereby delay fatigue."
Read the rest: Staying hydrated during exercise.