When it comes to staying hydrated Mark Verstegan, Founder of Core Performance recommends 1/2 – 1 ounce of water/pound of body weight/day.  For a 180 pound person that means 90 – 180 ounces – higher amounts on days with more activity or higher temperatures.  Being properly hydrated will increase your brain function, increase energy, increase focus, decrease stress hormones and help to control appetite.  Note that 1 litre is equal to 33 ounces.