For (european) low-cost airlines such as Ryanair,EasyJet,SkyEurope,Whizz,AirBerling,Stirling,Niki,AirLingus,etc. - the cheapest booking time is about 3 months in advance. They have a contingent of very cheap tickets that you can usually only get hold of if you book really early. But they do also have special promotions sometimes...
Edit: just saw that you asked for flights between Europe and the US...my advice was for flights in Europe...

Belgrade-Bucharest-Budapest-Brno
Edited 1 time(s). Last edit at 2006-02-06 14:28 by Lorenz.