Prague taxi drivers frequently overcharge. To avoid this, make sure that the hotel arranges your taxi--most of the time, the price will be fixed. Also, make sure that they accept credit cards (cos many don't).
They also drive like car thieves and maniacs, pretty much all of them. Bring your blindfold and your valium.
Prague shopkeepers very often count on you not recognizing the Czech coins and give you too little change.