In reply to James Malloch: I believe the Metro closes at midnight, so any bus route to town via Dejvicka Metro station will not be available to you. Line A, which Dejvicka station is on, closes down from midnight to 0500.
There is a bus service to the city centre - Námestí Republicky - from the terminal. Tickets are on sale inside the airport or you can buy it on the bus.
More info on the easyjet site here:
http://www.easyjet.com/en/cheap-flights/czech-republic/prague/airport-prg
You are arriving at a difficult time of day !!
My girlfriend has flown with Wizz Air. She said she had good service and they were on time.