Whenever my kids don't want to finish their juice box, for some reason they refuse to save it and finish it later on.  I don't want to waste the juice that remains so I take a sippy or a water bottle and put the rest of it in the cup.  Then I mix it with some water to make sure it is a full cup and it gets placed in the fridge for them to help themselves to it.  Voila, no more wasted juice and they never know that it's the leftover juice.