Hi there,
Actually I work at a licensing company and the way it usually works is artists come to us with their music and we decide if they go on our roster. Which we in turn work our butts off to pitch to music supervisors and film people. We then split any royalties from those placements 50/50 with the artist.
This is not that common but there are other places you can sign up your music with where there are lots of licensing opportunities. Some artists I have spoken with made a lot of money just by having a good commercial product with the right company.
I.e Pump Audio, though they split royalties now like 30/ 70 which everyone's crazed about. You can also sign up with You License where there are lots of banRAB and lots of licensing opportunities. I think Jingle Punks is another.. My company isn't currently signing. Most of these places its a non exclusive deal so you are free to do whatever with your songs and they don't own them.. Hope that helps!