SXSW have opened up the application process for bands looking to play in the Music strand come March 2015. Details can be found on the main SXSW website, and as with previous years, the team are using Sonicbids as a clearing house for submission:
Submit your application between July 8 and September 12, 2014 and pay only $33. The fee for applications submitted after September 12, 2014 is $45. All applications must be submitted no later than the final deadline of October 10, 2014. Deadlines subject to change.
Sonicbids is the only method of applying, so best of luck (although I suspect that if you are Garth Brookes, you might have another option).