From now until May 15, 2013 you can earn 2 Airtran credits for each one way coach flight (4 round trip ) or 3 for business (6 round trip) after you register and complete your flights by this date. Flights must be booked through Airtran and not Southwest website, however, after you earn these credits, they can be transferred to Rapid Rewards.
1 Airtran credit = 1 Rapid Rewards and 16 RR equals a free round trip flight which will be valid for one year from when the reward is issued. For black out dates see the T&Cs.
If your airport operates Airtran and has any good deals, then make sure to register!