Being fully flexible is the best way to find cheap flights. Flying in the summer will likely be more expensive than in the spring and fall. Be sure you first use search engines such as Skyscanner or cheapticket.com. They give you the otion to search flexible dates and nearby airports. If you want to fly to Amsterdam you might find cheaper flights to Brussels or Dusseldorf available.
Start looking for flights well in time before departure. The change of finding really great last-minute discounts on flights is really small, the earlier you book, the better fare you will find!
First you go to goeurope.com and start your search. You get the options for Trains, Buses and Flights all at once, making it easy to compare not only the fare, but also the duration of the offered option.
GoEuro has rates that you will not find if you book on the individual websites.
It's a great way to find out the option that you have to travel accros Europe.