Store leftover prime rib in an airtight container or tightly wrapped in aluminum foil in the refrigerator. It can stay fresh for up to 3-4 days. To maintain its juiciness, I recommend reheating gently in a low-temp oven (around 250°F). Place the prime rib in an oven-safe dish, and cover it loosely with foil to prevent drying. Heat until it reaches your desired warmth, periodically checking with a meat thermometer. Avoid microwaving, as it can lead to uneven heating and potential dryness.