I'm not familiar with the when to drink water, but I have read that ~80% of people are functioning in a dehydrated state. I have also read that our bodies don't differentiate between hunger and thirst. So, many times when we think we are hungry, we are actually thirsty. We also need to factor in activity levels and diuretics (like caffiene) - the more active you are, &/or the more caffiene you consume, means you need more water.
Another strategy would be to try and minimize drinking your calories (some juices have as much or more sugar than pop). You are better off eating fruit, than drinking juice - that way you get vitamins, hydration, and they help fill you up.
Just my $0.02