Thebeijinger.com, cityweekend.cn and beijing craigslist all have advertisements for housing.
58.com and ganji.com can be kind of useful depending on your Chinese level and your patience.
I would recommend checking the advertisements for seeking roommates on the Beijinger as it's relatively painless compared to the other methods.
In terms of your post, you'll also get more advice if you give people an idea of what part of Beijing you are aiming for.
Reply from 随便叫兽 is flagged as not relevant.
Having twouble finding a womb in Beijing?
www.58.cm and www.ganji.com, both is good to looking for a room