Consider trying the Travelocity Last Minute Deals search:
http://travelocity.lmdeals.com/
It's usually only good for weekend flights (travel on Thursday-Tuesday), but it can often times come up with very reasonable prices. For example, last Thursday I was looking at going MSP - JAX departing Friday...less than 24 hours in advance. Last Minute Deals quoted me something like $290 including a car rental. NWA.com and other travel sites were quoting me at least twice that for a less than 24 hour advance purchase.
Hope that helps.