Mix up to 24 hours in advance and refrigerate for improved flavor. Bake just before serving. Store leftovers covered in the refrigerator for 3 to 4 days. Reheat in a 300 to 325°F oven until just hot and creamy to avoid breaking the dairy, or microwave in short bursts, stirring between. Freezing is not recommended as sour cream can look grainy after thawing.