audiosparx.com, sounddogs.com, etc. are ways to sell sound effects and music online. Music Supervisors are tasked with providing choices of music to go into advertisements, television shows, or films. I say choices because ultimately the higher ups usually have final say although a good Music Supervisor's opinion is highly respected.

Various performing arts organisations (PROs) like ASCAP and BMI are faced with the task of collecting your royalties. Audiosparx has some really good information about the various types of licensing deals that are possible in their "terms and conditions" when you sign up for the site. It is free. Audiosparx can also help guide you through joining a PRO.