If you register with Southwest Airlines website, they send out what they refer to as "dings" every morning. I think it may be their way of filling the unused seats on flights.
When you recieve a "ding", you have a very narrow window in which to respond. And of course, the nearer you get to your departure date, the price goes down. Conversely, if you wait too long you run the risk of the flight being filled.
We travel to Las Vegas quite frequently, and always get great deals. In 2005, we flew round trip, Omaha to Vegas and back(non stop I might add), for 108$-for both of us!! Of course we waited until a week before we were leaving to buy our tickets, so that was cutting it close!!
We are going again in a couple of weeks, and our tickets this time around are 227$ each-still a pretty good deal, I think. Since my wife normally takes care of this, there are probably details I'm overlooking, but it is worth checking out.