as they said, drink lots and lots of water. you body isnt going anywhere without water. usually, more water means that your body functions better in just about every way.
one thing i think should add: to be properly hydrated, one needs water AND electrolytes. normally, lack of electrolytes is not a problem. you usually get sodium from food, so, drinking lots of water will rehydrate you...however, if you are out in the sun all day, or in some other condition where your body's hydration is highly strained for a long time (and you are sweating a lot), drink a little bit of gatorade, or something with electrolytes, followed by a lot of water.
it is possible to drink too much water...however, to do this, you need to drink an absolutely ridiculous amount of water...i wouldnt worry too much about that...ive downed 2-3 gallons in a day on occaision with no problem.