1. Sign up for fare alerts with Farecompare and/or Airfarewatchdog. Keep in mind that sale fares don’t last forever, sell from a limited supply of seats, and are not always the cheapest possibility. So when you see something really good, I’d say in the $500s for this, you need to act fast. Compare your routes with consolidators (“unpublished” fares) to be sure you’ve got the best offer for your itinerary.

2. You can get overview snapshots of what the price is currently for those dates by using ITA and Farecompare. There’s an ITA tutorial in our new user thread (@ top this forum). This Farecompare link will let you see fares that airlines have filed from your city; you can adjust outbound month there. Use airport code WAS to cover IAD, BWI, and DCA all in one go.

With total flexibility, I use Farecompare to see what’s been filed, then I try those cities with ITA to find actual itineraries at those prices, then I book direct from Orbitz or the airline’s own site. Note that on rare occasions, airlines will file a fare that they don’t actually allocate seats on (by mistake?).

3. Sometimes travel on Dec. 31 is cheaper; ITA’s month search will let you see price differences depending on departure dates.

4. Ryanair is not always cheapest. Their fees can kill the deal if you don’t plan well. Use ITA to check for “big” airlines on the route—sometimes they’re competive on price when all is considered. Use Skyscanner and Momondo to check intra-Europe flight possibilities, too. Momondo sometimes turns up train tickets in it’s search so you can see how it compares.

I would also consider all WAS airports, PHL, and maybe even NYC-area airports. I would say that somewhat more than 50% of the time in the past year, it’s been cheapest to fly to a German city, or maybe an Irish city. Like Don said, FareCompare and ITA are great.

Also, for intra-European travel, look at www.whichbudget.com . Using this, I see you can get a $485 all-in roundtrip flight from IAD to Moscow, for Dec. 27 – Jan. 17. Of course travel to Russia is a whole other ball of wax with visa requirements, etc., not to mention going there in winter.

You can do PHL-FRA (Frankfurt) for $603 all-in, also on Vayama, for your exact dates. But I’d try to work out an open jaw, even if it’s more expensive. But even if you can’t get an open jaw, Frankfurt has innumerable onward transit connections, and it’s very central for the trip you want.

The reason I suggest other cities for your departure airport is because I know Megabus is a cheap and fast way to get between these cities, although I’d definitely go the night before, as Megabus can be delayed (that’s what happened to me with an accident on the New Jersey turnpike that the incoming bus could not get around).