Allow meatballs to cool to room temperature. If you plan to serve the meatballs within 4 days, store them in an airtight container in the refrigerator. Be sure to separate layers with parchment paper to prevent sticking. If you're making them well in advance, you can freeze the meatballs. Place them on a baking sheet in a single layer and freeze until solid, then transfer them to an airtight container or freezer bags. They can be stored in the freezer for up to 3 months.