Southwest doesn't do coupons very often anymore. Most of it is done by email and DING - look up their website and sign up for their email alerts and download DING - DING will send you updates daily for the markets you want to fly to and from. If you purchase tickets now at the rates you see (which tend to be the cheaper when they first come out) and if you find a DING rate, you can call Southwest or do it online and get the difference between the rate you paid to begin with and the sale price in credit towards another flight.
Hope that helps.