Prague's airport Ruzyne has recently been rebuilt, refurbished and expanded. It is about 20 km northwest of the center, unfortunately there is no direct tram or tube service however you can take a regular public bus service (191 to Dejvicka or 179 to Petriny) or a taxi.
Express airport bus (welcome Touristic or Ceda bus) runs every 30 min from the airport to the town. First stop is at Dejvicka tube station, line A and next stop is at Revolucni Trida in Prague 1. The twenty minutes ride is not expensive, price depends on were you want to drop off.

Most of the worlds major airlines operate in and out of Ruzyne airport, or if not directly then via shared services. CSA is the main airline but there again it is the national airline, CSA now operate on a global scale with the recent inclusion of direct flights to the USA.
From the UK CSA, Go and British Midland are your cheapest bets check the prices currently offered, return flights start from about £100 but there are always some sort of special offer around.
To the right is the contact details for all the main airlines in Prague and some of the smaller ones.

"Airport Taxis" are only allowed to operate between the airport and your destination, and are the only ones allowed to sit waiting to be hailed outside the airport. However this means that there is a catch they are considerably more expensive than standard taxi's with a one way ride into the city will set you back about 750CZK.
These taxi's can be booked in advance or hailed outside as they wait in ranks aware that the unsavey prey is theirs.
These cars carry adds (descrete) for Visa, however it should be note that in the tradition of ex-Eastern Europe they only accept cash.

However if you wish to save a few pounds why not call for a normal cab to come and collect you either prior to departure (obviously giving arrival and flight details) or while you wait to go through customs. The city center is only 15 minutes away so you should not have much wait. A normal cab will cost you about half as much.