I've eaten duck at a few places, including Xiaowangfu's with moondog about a week ago, and Xiao Wang's is excellent. However, the best is a restaurant near the Andingmen Subway stop on the first floor of a hotel. I'm blank on the name, but will look it up and post the info.

P.S.: 室雅人和 Restaurant (Shi4 Ya3 Ren2 He2) near Andingmen 安定门 next to Ditan Park 地坛公园
Address: 地址：朝阳区安定门外大街188号　- Open until 10pm