If you're cooking them the same day, prepare the meatballs and place on the baking sheets. Cover with plastic and stash them in the refrigerator until you're ready to cook. For longer storage, you can freeze the meatballs raw or cooked. Either way, freeze them on the baking sheets to prevent them from sticking together, and once frozen, transfer them to a resealable bag and store them in the freezer for up to 3 months. When baking or reheating frozen meatballs, your cooking time will increase.