8 regular sized tumbler's is a good indication of how much to drink, but if you drink plenty of tea, coffee and squash your 6 glasses will be plenty, your normal every day drinks should be part of your daily allowance, but I drink a 2litre bottle of sparkling water extra to my normal drinks, just because I love drinking water and have done so for many years.
It's not only good for your diet, but it helps your skin stay nice and soft too by keeping it hydrated, if your skin dry's out, that's when wrinkles start forming, it also keeps our kidneys working properly too, so water is very good for us - all of the time.
I drink 2 litres of sugar free squash a day and I really notice it when I don't - it really helps with bloating and water retention as when you don't drink much your body trys to retain as much water as it can.
I have drunk 2 litres of water this week and can say it has helped my weight loss as well as the other benefits that are mentioned above. I keep a drinking bottle on my desk and try to drink at least a litre whilst working at my desk.