I'm just going to get straight to the point:
The idea that you need to drink 8 glasses of water a day is an absolute myth. The amount of water we need to intake varies from person to person. Here are some of the variables to take into consideratoin:
Body size
Climate
Health
Diet
Amount of exercise
Gender
Age
Weather
etc...
To think there is some rigid structure of how much water any person should intake is ludicrous. Drink if you're thirsty. If your urine is dark, drink more water untill it clears up. If it doesn't, then its a good idea to consult a doctor because you may have a health condition that requires you to drink more.
Also take into consideration the fact we take in a large portion of our fluids from solid food.
I have heard this but I think it's better to have more than not enough. I think that to many people just get a soda or some other unhealthy alternative when they are thirsty and go for days without actually drinking any pure water. It can cure a lot of things so I think it's a good first step if you're feeling sore or like you may be getting sick. If you develope something like that from not drinking enough then I think it will take a little longer for your system to correct itself.
The sick feeling that people get from drinking too much soda is probably a result of either way too much sugar or sodium, not that they get dehydrated. There
is such a thing as drinking too much water, though its quite excessive (drinking more than 1 liter of water in an hour can lead to minor intoxication as your kidneys will start to get overloaded), and would take quite the effort to get that far. In fact, the USDA recommends 8 glasses of fluid each day (the human body loses about 3 liters of water daily, but remember a lot of things we eat or drink has some amount of water in it.) People automatically thought they meant 8 glasses of water, when in fact an average of 4 of these glasses of fluid actually came from solid food by itself. Its perfectly viable to survive on just food and soda for water in take, though its not exactly healthy because of all the sodium in soda (go figure). Juice is a great alternative.
Drink when you are thirsty, avoid soda in favor of juice and water, eat a balanced diet. Keep your diet simple. Don't worry about counting how many ounces of water you drink a day and counting how many calories you intake. Thats just too much effort for it to actually be worth doing, why should you spend hours of your life away trying to maximize your life span when it could very well be taken away by a drunk driver running you down while you're walking to the store? Enjoy life for what it is,
not a race to be the oldest living human being.