How much water are you supposed to drink every day?

Most of the information I've seen on the topic recommends that you drink 8 or 8-
10 cups (8 ounce servings) of water per day.  Not drinking enough water can 
cause numerous negative effects.  Headaches are probably the best known effect 
of not drinking enough water, but other things like heartburn, sore muscles, 
high blood pressure, kidney stones, and fatigue can be either caused or 
worsened by not drinking enough water.  This page at healthforge.com mentions 
that the recommended amount of water consumption is 8-10 glasses per day, and 
describes in detail the ill-effects of dehydration:

http://www.healthforge.com/structured_water/prevention.htm

Another page you may find interesting is this summary of the drinking habits of 
Americans by the International Bottled Water Association.  They discuss a 
survey which showed that most Americans drink less than the recommended 8 
glasses of water per day.

http://www.bottledwater.org/public/InfoForRepNatFactSheettest.htm

Here's another article about water consumption that recommends you drink 1/2 to 
2/3 ounces of water for each pound of body weight per day:

http://wmi.nols.edu/buck.html

I hope this answers your question!

Very detailed, thanks!

According to this CNN article, the "eight glasses per day" quote is a
myth.  Scientists are only just now beginning to conduct a study on
the matter.

http://www.cnn.com/2002/HEALTH/diet.fitness/05/24/cohen.water.otsc/index.html