Try to always negotiate a price with taxi drivers. Never take a taxi directly from Wenceslas Square, Old Town Sq. or Mala Strana Sq. Cabs that are allowed to stand on such places belong to mafia (Russian or Bulgarian) and cabdrivers will rip you off badly. When necessary walk couple of hundreds meters in any direction and take taxi from there. A typical 5 min taxi ride downtown ranges from 150 to 500 CZK depending on you ! Many restaurants and hotels pay commission taxi drivers. Be aware of that and try to find a taxi yourself. Don’t forget that the Metro and Tram System in Prague is good and inexpensive.
TIP: Always discuss the price with the taxi drivers before going anywhere! A friend of mine had to pay about 900 crowns alone instead of 500 we paid in 4 persons.
We negotiated a price to\from the airport and that worked out OK, but beware, another taxi we caught was on the meter and it was like £30 for 5min journey across town. As most people say, avoid taxis if you can!