The number 100 bus is signposted in English from the front of the airport terminal. Tickets can be purchased from the machine next to the stop; catch the bus to Zlicin at the end of the yellow metro line, and change onto the metro. From there to the centre of Prague. All signed in English. Easy and cheap; feel smug about how simple it all was afterwards!
Airport to Zlicin metro via 100 bus
Airport to city centre (to Metro A - Dejvicka) take Bus 119