A good rule of thumb for staying hydrated:
Drink half your body weight in ounces. Every day.
Example: Say you weigh 100 pounds. Well, drink 50 ounces of water every day!
Another way to make it easier on you: invest in a nice big water bottle (BPA free, environmental friendly, all that hippy stuff) that you only have to fill up two or three times a day. Makes it a lot easier to keep track of your ounces!
And remember, it's best to sip your water throughout the day rather than chug it all at once. Sipping it is easier on your system overall ( your kidneys especially) and your body absorbs the water much easier, therefore giving you the full benefits of hydration!
Happy Drinking! (water, that is!)