It sounds simple but how to do that is the subject of much debate and how much water one drinks and needs to drink and how to drink it apparently has a lot of different opinions as I found out when I went online to try to figure out what the answer was.
Your body can only absorb so much water in one shot and above a certain level, it just runs through you and you pee it out. So drinking a gallon of water once a day is not a good idea. It's tough to do, but for some people, doing things like that can cause your sodium levels to drop because it dilutes the concentration in your blood.
On that matter, everyone varies based on their body size and activity level, but the average guy needs about a gallon a day of water and the average woman needs close to 3/4 of a gallon.
Drinking small amounts throughout the day seems to be the right idea. Use your urine color as a dipstick - it should be mostly clear and not too much too frequently.