A new Tunecore Soundcheck poll shows that a surprising 44% of musicians do not protect their work by copyrighting it.  Reasons varied from "I don't know how. to "cost ($35 and up) to "It's too expensive" and "I don't see the need".  Poll results and resources on how and where to copyright your creative output: 

SoundCheck Results: Copyrighting Your Music

Results to the poll: Do you register the copyright for your music?

Resources:

• U.S. Copyright Office forms and instructions.
• Article: How To Copyright Your Music
• Creative Commons