You can often find a great deal, especially at short notice, if you're willing to buy tickets without knowing your airline or exact flight times. Opaque pricing sites like Hotwire and Priceline offer deals of this kind.

Hotwire allows users to save on both domestic and international tickets. It works with major airlines to offer their unsold seats at a discount. You can get a sense of how good the deal is by comparing the price Hotwire offers you with the lowest published fare on the route.

If you want to name your own price for airfare, Priceline is another type of bidding site that allows you—and not the airline—to set the price. The site's "name your own price" feature allows you to bid for rock-bottom prices on airline tickets, without knowing your airline or exact flight times (departures can be any time between 6 a.m. and 10 p.m.). You also must be willing to accept at least one stop in each direction. If priceline accepts your bid, you're locked into the fare. To make sure you don't over-bid, you should check the lowest published fare first, and bid about 20 to 30 percent lower. You'll know within 15 minutes if Priceline has accepted your bid; if not, you can re-bid by changing one of your choices, or go back to Hotwire and purchase your ticket there. Your total cost on Priceline is your accepted bid plus airfare taxes and a per ticket processing charge, so be sure you are comparing the total amount to see which site offers you a better deal.

If you need more information, Priceline also allows you to choose your flights, times, and airlines, at higher rates than their usual rock-bottom fares.

With both Hotwire and Priceline, be very careful on a two-day trip, because you might have to depart late in the evening and return early the next morning. Also, note that with both sites, one-way travel is not available, you won't earn frequent flyer miles for your travel, and your tickets are entirely nonrefundable and nonchangeable.