You might also consider bidding via www.priceline.com . It doesn't always work, but occasionally someone scores a good deal.

Caveats:

No choice of airline
No choice of time of day
No refunds

You can start bidding as low as you like. Lots of info and technique suggestions on www.betterbidding.com .

Start by researching the very best rate, and decide how much discount you need to be worth it for you to not be able to choose a specific flight.

Romelle