We always fly SWA and as soon as flights are released for our travel dates, I price them out and watch them daily as they do change often. Once I find a fare I'm comfortable with, I purchase the flights but I keep watching the fares because if they happen to go down, I can get a credit to use on a future flight (which has to be used within a year from original purchase date). Some people find SWA fares to be the lowest on the day are released, but for us (flying out of Chicago), they usually always go down after they are released, so I don't buy right away.