For recreational exercise, drink to thirst, but be proactive if you're racing.
In his new book 'Waterlogged,' Dr. Tim Noakes (of 'Lore of Running' fame) discusses the "dehydration myth" propagated by the sports drink industry: That athletes should hydrate to the point of never feeling thirsty, because thirst itself is supposedly a sign of dehydration. The dehydration myth, Noakes writes, created a culture of overhydration, which can cause exercise-associated hyponatremia (EAH), a potentially fatal condition caused by excessive water consumption and electrolyte loss. While Noakes suggests that "drinking to thirst" is always adequate, hydration expert Doug Casa says the principle is really only good for low-intensity, recreational exercise. For more competitive athletes, Casa says "if you're only drinking to thirst, you aren't being proactive according to what your needs are." This means starting off hydrated and drinking small and steady amounts throughout the physical activity.