I'm not sure if anyone has discussed this before, but I think it can save us some money in our neverending quest to save a few dollars.
Southwest Airlines (SWA) offers something called DING! fares. It goes like this: you install a program on your computer. Periodically, the program makes a DING! noise (two to three times a day I've found) and this means they have an airfare sale. The airfares aren't always for your home airport, or not even necessarily where you need to go, but you can check very quicly which sale fares they are offering. These fares are about 15% off the normal fares they offer, and must be booked two weeks in advance. I have seen the normal San Diego to Las Vegas segment, a $39 fare, down to $27. The fares are only offered for a few hours, so you have act while they are available and still valid. I read the DING! web page whenever the fares are offered to see what the fare sale is, even if I'm not currently looking to buy a ticket.
With Southwest, you can always book the flight under the lower fare, and if you need to change your reservation later, they will happily apply the fare you paid on the DING! fare to any other fare, just like their other tickets.
More information, including links to the program you can download, are available at the following address:
http://www.southwest.com/ding/
--DB in SD