I love my pink 24 oz CamelBak water bottle. It's marked at the 8-16-24 oz levels, making it easy for me to see at a glance how much water I have drank from the bottle. I keep it filled with ice water on my desk at work and have a goal to finish 3 bottles per day (72 oz or 9 cups).
Of course it doesn't have to be a CamelBak bottle - any bottle that is see-thru and you enjoy drinking from will work! I even have a special koozie ordered that will help keep my water even colder.