Which fluids should I be drinking?
According to the American Dietetic Association’s Complete Food and Nutrition Guide, the average adult loses about two and a half quarts or about 10 cups of water daily (those living in hot climates or those that are physically active may lose more). You will need to replace these losses with adequate fluid intake. Various types of fluids count towards your needs for the day (water, juice, soda, milk, smoothies, soups, etc). Even caffeinated beverages can count (coffee, tea, cola). Caffeine is considered a diuretic. However, research shows that when your body is used to the caffeine, you may not lose much water at all.
I generally recommend that one monitor how many calories she is drinking, as consumption of excess calories can lead to obesity. It sounds like you are already choosing diet or low sugar products, which is great. Another good example of a low calorie way to increase fluid intake is Healthy Balance Juices!