Taxis are plentiful, easy to spot, and by far the most comfortable way to get around Beijing, though increasing traffic means they're not always the fastest. There's a flag-fall of Y10 for the first 4 km (2½ mi), then Y2 per kilometer thereafter. After 11 pm flag-fall goes up to Y11, and there's a 20% surcharge per kilometer.
Drivers usually know the terrain well, but most don't speak English; having your destination written in Chinese is a good idea. (Keep a card with the name of your hotel on it for the return trip.) Hotel doormen can also help you tell the driver where you're going. It's a good idea to study a map and have some idea where you are, as some drivers will take you for a ride—a much longer one—if they think they can get away with it.