Reserve the duck fat. (It will keep 3 weeks in refrigerator or 3 months in freezer; reheat and strain before using). If you're re-heating the duck legs, add a tablespoon of the duck fat to a heavy skillet and heat over medium high heat. Once hot, place legs skin side down to crisp the skin. This will only take a couple of minutes. I also toss some more garlic cloves and thyme sprigs into the pan while I'm cooking the legs. Can you ever have too much garlic?