First of all you want to start out by determining how much water your body needs. A great equation to use is:
Your body weight multiplied by .67 (2/3)
So if you weigh 150 lbs you would need about 100 ounces of water a day.
You must also factor in how physically active you are in the day. A good rule of thumb is to add about 12 ounces for every 30 minutes of activity you expend.
So for example, that 100 ounces of water for a person weighing 150 lbs will increase to 124 ounces if you workout for an hour in the day.