One method I've employed twice before is if you have live leftovers you can put them in a plastic container and put them in the freezer. Call me inhumane if you want, but it's the simplest way to preserve them for another feeding day. They will get cold, their fast heart rate will decrease, they'll go to sleep, and finally die and be preserved at their freshest for later. That or you have a whole other cage for rodents complete with bedding, food, and water. I couldn't do that without other household members getting attached to it, and I didn't need another pet.