Here is 3 highly recommended choices.
1.Quan Ju De Roast Duck Restaurant(Wangfujing Branch) established around 1864
2.Da Dong Roast Duck(face to Great Dragon Hotel)
3.Bian Yi Fang Roast Duck Restaurant(1 floor of Hademen Hotel,Chongwenmen Subway station)
Here is where I got these from; http://www.tripadvisor.com/ShowTopic-g294212-i2147-k1849148-The_best_Peking_Duck_Restaurant_in_Beijing-Beijing.html
Good luck with your duck hunt! :)