Hydration

"Six glasses a day" was (and still is) a phrase oft-repeated when discussing proper hydration. The evidence for this amount (how much is in a "glass", anyway?) is pretty thin. That aside, though, how many people are going to keep count?
Here's an easier way: every time you urinate, go replace what you lost by drinking something, preferably plain water. Doesn't matter if you're thirsty; thirst is an unreliable indicator of dehydration, especially the older you get.

It's not just about water, though. It's about the stuff dissolved in it, too. "Pure" water (just H2O) is actually pretty hard to come by, and not very good for you, either. Commonly available water filters don't actually "purify" water, just remove a lot of things you're mostly better off without. A carbon filter for your drinking water is probably a good investment.