I would try one of two strategies, depending on where you need to go.
Frontier, ATA, and Air Tran have reasonable fares that do not require an advanced purchase, it only depends upon if they have the seat available for sale.
Also, check out priceline.com. You will NOT get frequent flier miles, but you may be able to fly cheap at the last minute. Use the Priceline EzBoard for bidding strategies.
http://pub4.ezboard.com/bpricelineandexpediabidding