I don't recall exactly how or when I came across the one-third rule, but I've been using it for years: basically, almost anything with a liquid base (and even some things, like re-fried beans, which aren't liquid) can be diluted with plain old tap water by 1/3 without affecting taste or quality all that much. Examples: orange juice--in fact, I can barely drink the stuff straight from the carton anymore. Soup. Ketchup. Salad dressing. Balsamic vinegar. By adding 33% more volume, you automatically reduce the calories/carbs by one third as well, for a save calories-save money, win-win proposition.
The 1/3 rule works for non-edibles as well: Dishwasher liquid, laundry detergent, bleach, window cleaner, ammonia, body lotion, shampoo, conditioner--the 1/3 rule works with any number of common household, cleaning, and grooming products, and is a great way to painlessly reduce spending.