All canceled Southwest Airlines tickets qualify for the company's "Reusable Funds" program. This means that 100% of a ticket's value can be applied toward the purchase of another Southwest Airlines ticket. The funds must be used within the 12 months following the cancellation.
You can cancel a flight reservation made online or over the phone here or by calling reservations at 800-435-9792. Reservations made through Southwest Vacations, Southwest Groups, Southwest ticket counters, and a partner airline (such as AirTran Airways) can only be canceled by calling 800-435-9792.
Of interest, Southwest recently linked its booking system with AirTran Airways. As of April 14, 2013 fliers of AirTran or Southwest will able to add a connecting flight on either airline in the same transaction. AirTran passengers must still pay fees to change reservations and to check bags.