I reheated half a Peking duck from Chinatown yesterday. I covered the cut side with aluminum foil to prevent moisture loss and catch any juices, then put it in the oven preheated to 400F for 15 minutes or so. I placed it on a pizza stone, as I had heard of someone else doing, but I doubt this made a difference - perhaps with a whole duck it would help. The skin was crisp and meat moist. Wikipedia describes various methods such as reheating it by dipping the duck in hot oil, but this seems messy and more complex than needed. Good luck.