In the case of the airport and/or hotel taxis, the keys to not getting ripped off are (1) to confirm that the meter has been reset when you get in and (2) get a copy of the receipt (and the driver's name and license number if necessary).

Of course, if the driver takes a roundabout route, it is hard to know if you are new to the city. However, don't freak out also just because you're on some highway that you've never seen before. Beijing has five (or is it six now?) ring roads, and taking them can sometimes be a faster albeit longer route.

At the end of the day, taxis in Beijing are so cheap anyway...it is not worth getting too stressed over it.

Oh yeah, a few years back, I was ripped off by a taxi (which was waiting outside the Holiday Inn Crowne Plaza -- a foreigner hotel if there ever was one) on an airport ride. The taxi driver was really friendly and took the normal route. Only when we arrived did I discover that the fare was 3x what it should have been. I challenged the driver but without concrete proof other than the ridiculously high fare, he denied any wrongdoing (surprise, surprise) but agree to accept less than what was shown on the meter. Still, it was about 30 kuai more than it should have been. In hindsight, I should have given the driver nothing and walked out and threatened to call the police if he tried to stop me.