Drink when you're thirsty.

What a relief, I can stop worrying that I don't drink enough water.

Myth No. 1: Drink Eight Glasses Each Day
Scientists say there's no clear health benefit to chugging or even sipping water all day. So where does the standard advice of drinking eight glasses each day come from? "Nobody really knows," says Dr. Stanley Goldfarb, a kidney expert at the University of Pennsylvania.

Myth No. 2: Drinking Lots of Water Helps Clear Out Toxins
The kidneys filter toxins from our bloodstreams. Then the toxins clear through the urine. The question is, does drinking extra water each day improve the function of the kidneys?
"No," says Goldfarb. "In fact, drinking large amounts of water surprisingly tends to reduce the kidney's ability to function as a filter. It's a subtle decline, but definite."

Plus more myths here.