There is at least one silver lining to the economic downturn: There are going to be bigger airfare sales this year, and they’re going to start earlier than usual. Fearing a possible sharp decline in commercial traffic in 2009, the airlines are desperate to fill seats on their planes this holiday season, so they are going to be offering rock-bottom prices on flights. Rick Seaney, the chief executive officer of Farecompare.com, has said, in fact, that the sales will be the biggest the airline industry has seen in the past 18 months.

So how do you find the fare sales? You have to shop around. Websites like Farecast.com and Airfarewatchdog.com are a good place to start, but also check the airlines’ websites—they often offer discounted rates that you can’t find on booking engines like Travelocity and Expedia. For example, Northwest Airlines has just announced a holiday fare sale on its website, with tickets as low as $180 for roundtrip travel between Boston and Detroit, and $160 between Indianapolis and New York. United also has an international holiday fare sale on its site, including roundtrip tickets between New York and Munich for $500 and roundtrip tickets between San Francisco and Rio de Janeiro for just $840.

Another tip: Be flexible about when you fly. Traveling on what the airlines call “super off-peak days,” such as Mondays and Tuesdays and the holidays themselves, can save you tons of cash. For instance, the best days to fly at the end of the year are Christmas Day, New Year’s Eve, or New Year’s Day—the priciest days are Dec. 26-28, and Jan. 2-4. Travelocity, Orbitz, and Cheaptickets all have flexible-date search functions, which allow you to search for flights on the days before and after your preferred day of travel.