My advice is to start getting an idea of what flights are available, which airlines fly non-stop, etc. Expedia, Travelocity, et al are a good place to start that research, but not a good place to book, IMO. It's also important to note that not all airlines, including SWA, show up in your searches on those travel sites.
Find out what prices are running, figure out what you want to pay, and when the two meet--book the tickets! But go directly to the airline's own Web site to do it.
FWIW, SWA opens bookings through March 12 on September 1 and probably will get around to opening at least some May dates in November (no guarantees, however). It could be January before all of May is open, however, so you have to determine your tolerance for playing the waiting game if you have other options that fit your
budget.
Don't forget to consider the "extras" when pricing your tickets. These days that includes baggage fees (most airline charge you at least $15 to check one bag and more for additional bags, SWA doesn't currently charge for the first two bags), assigned seating, etc. All of those can add up.