Beijing buses can get very crowded so be prepared and keep an eye on your valuables. Many pickpockets frequent buses and subways, so carry backpacks in the front, and try to put your valuables somewhere hard to access.
Be aware of a scam offering bus rides to the Great Wall masquerading as the real bus service. Instead of directly driving to the Great Wall, you will instead be led to a series of tours to dilapidated theme parks, tourist shops, museums, etc before finally reaching the Great Wall near the end of the day.