If you're on the way to Prague and you already have a map of the city's excellent tram and metro system...you should get a new one. And if you do get a new one, be sure it really IS new!
For a start, the Metro line A that connects to the #119 bus that connects to the airport has been extended four stops closer to the airport. All your instructions are likely to tell you to change from bus to train at Dejvicka; on the bus they will drop you at the new Nemocnice Motol station.
Also, the most popular tram maps need updating (including the one that's sold for about 8c at the Tourist Information) are behind the times on a number of tram-line changes. To avoid finding yourself at a transfer point no longer served by the you want, download the latest from the Prague Transit Company HERE.
By the way, here's the trick to using the tram system. At every stop, there's a sign with the name of the stop on top, and below it a list of stops for each line that stops there. Just find the stop you're at (it's underlined) and see if the stop you want is listed BELOW it. If so, you're in the right direction; if not, cross the tracks and take the tram in the other direction.