Be safe, but be realistic
It's true that the "poor man's copyright" trick of mailing it to yourself is fraught with uncertainty at best. However, given that the odds of someone liking your material enough to steal it are pretty low, it's difficult to justify forking over 30 clams to the US copyright office every time you write a song. Even more remote is the likelihood that someone would be able to make enough money off of your song to be worth suing, but hey, you never know! That said, it's still silly to not copyright your material if you're putting it out where anyone can access it. If you don't want to go the legal route, by all means mail yourself a copy. Load a copy onto your hard drive and be sure that it's dated. Pass copies around to friends who will attest that you gave it to them on a given date. In other words, do everything you can to protect yourself at the financial level you're best able to afford. But be realistic about it. IMHO, if you're just passing out a home-made demo to your friends or a local club owner, spend the 30 bucks on something more tangible. If you are seriously releasing material over the Internet, radio or retail, not only should you get it copyrighted, but it's also wise to register the performance rights for your song as well. This protects you from other people playing your song live, and passing it off as an original. Most of us would probably be flattered if someone thought our songs were good enough to play, but that doesn't mean someone else should profit from your talent and work! The performance copyright symbol is a captial 'P' with a circle around it. I put one of those on every disc I make, even the one-offs, along with the © symbol, the year, and the words "All Rights Reserved." Like a previous poster noted, a written version of the song isn't necessary. I believe the form the copyright office uses is called the 'SR' form, for "sound recording."