The 8 glasses per day figure was made up (I'm sure it was by Robinsons, or someone
) in order to try to get people who do not drink enough to increase how much water they consume. There is no point in sticking a random number at it though, as it depends on so many things (your size, amount of exercise, what you eat...) Other drinks and water in food also count towards your daily amount too.
The way I look at it is that I should never become thirsty. That doesn't mean drinking litres of water in a day, making sure I don't become thirsty. It just means having a glass on my desk and taking a small drink from time to time as a matter of habit.