Most of the time I have some song stuck in my head. If I'm not playing music, I'm "listening" to it by brainwaves.
Example: I'll have some iTunes playlist going on shuffle while I'm working, but not paying direct attention to the music. Then I'll get in the elevator and discover I'm humming "Rosalia de Souza" by Maria Moita. How did THAT song get in my head? I'm never sure, but it was probably the last one playing when I hit that sleep button.
There are two good ways to get rid of a song stuck in your head: listen to another song and hum along (refreshes the cache) or sing a few notes of the stuck song to someone else, say "Tag, you're it!" (mental transfer of a stuck song).