Public Transport:
Buses of Prague Public Transit Co. stop at both terminals. A 75 min. ticket can be bought for CZK 26 at arrival hall (CZK 30 from the bus driver).
119: terminates in 24 minutes at the "Dejvická" station. Transfer to Metro line A to get to the centre. The ticket is valid on the Metro too.
100: terminates in 18 minutes at the "Zli?ín" station. Transfer to Metro line B to get to the centre. The ticket is valid on the Metro too.
179, 225: terminate in 45-53 minutes at the "Nové Butovice" station. Probably of no use for a tourist.
510: a night service every 30 minutes. Goes to the south of the city, but passes near the centre ("Jiráskovo nám?stí" or "I. P. Pavlova" stops) which takes 42 minutes.
A Czech Railways public bus service:
AE - AiportExpress - the bus connects Terminals 1 and 2 with Praha hlavní nádraží (Prague main rail station) every 30 minutes. The journey takes 40 to 50 minutes. Tickets are sold by the driver for CZK 50. You may also use any regular train ticket issued from/to station Praha letišt? (NAD) (bought in advance at some rail station or online), which is useful when you continue from Prague by train to some other Czech town.
Other buses:
Some local buses from Prague to Kladno stop at Terminal 1. You can get to Kladno for around CZK 30. See the schedule.
Student Agency buses link Terminal 1 with Karlovy Vary (direct, CZK 150), Brno (transfer required, CZK 250) and Jihlava (transfer required, CZK 195). Having a ticket in advance is highly recommended.
Taxis:
There are two taxi companies officially authorized at the airport. Both Radiocab taxi and AAA RADIOTAXI should take you to the city centre for around CZK 500.
Note that taxi fares are regulated by the Prague City Council. No taxi should charge you more than CZK 40 per ride plus CZK 28 per km. Taxi meter should be turned on and print a receipt.