I have lived in Prague on and off for 13 years. Go to the AAA Taxi desk in the arrivals hall and they will arrange a taxi. It is about 700Kc to the IP Pavlova or Namesti Miru area. It'd be a bit less to Old Town, and a bit less still to Smichov or Mala Strana.

Never just grab a taxi on the street or in front of the airport or train station.