If you are going to eat the leftovers within 3 days, I recommend storing the meatballs in the gravy in an airtight container in the refrigerator. If you want to freeze the meatballs for longer storage, make sure they have cooled completely before packaging for the freezer. I recommend refrigerating them overnight. The best packaging for freezing is a vacuum-sealed bag and make sure to freeze the meatballs in the gravy as the fat from the gravy will protect the meatballs from freezer burn.