They have a meter and ALWAYS stick to the meter!
Avoid the taxis on the airports and bus stations that stay there and give a "fixed" price for the journey since it is always more than the meter would show! Try to get the yellow ones that are stoping to leave passenger in the front of the stations and/or departure areas.
About the "drive around" thing written above, it is not that common anymore (I'm from Rio). And you can simple avoid that by taking cabs from companies instead of anyone from the street. Try to get the phone of a taxi company such as JB Taxi before you leave for the night out! about 9 years ago
In Brazil, they have a meter ("taxímetro"), and the previous answers are right, you should generally stick to that. Companies or Cooperatives are safer and will give you the correct price (by taximeter).
Search the WWW for "Taxi & Cooperativa & (name of city)" and you should get some numbers to call.
If you travel with luggage, they can charge you some 5R$ to 10R$ extra, or expect a tip from you.
Bargaining the taxi price is not common in Brazil. This said, in some cities, such as Belém, if you know the usual price for a certain route well, you can try to bargain, offering 75% of the taximeter price -- some drivers will accept at least 80%, 85% or 90% of the normal price. But ask lokals whether that is usual in the place where you are staying, in most cities (Rio, São Paulo, Salvador) this will not be accepted, in particular not from strangers. about 9 years ago
In Rio, the cabs are usually yellow. In São Paulo, practically all the cabs are white. In this two cities all cabs have the word "TAXI" well printed in the side.
And everybody here use the "taxímetro" to regulate the price, this is the correct and it's a law. about 9 years ago