Is it true that in order to get something copyrighted all you have to do is mail a copy of it to yourself?

When you mail something it gets a postage date stamped on it, and as long as the envelope remains sealed you would be able to use that postage date as proof of when you wrote the piece. As the previous answer says, this is "the poor man's copyright." If you were able to publish any of your poems, the publisher would be able to help you with the actual copyright.

Actually, copyright is even easier. When you write something, you have the copyright in it. You don't have to do anything. In the "old days", there was a difference between state (common law) copyright and federal copyright. If a work was unpublished, the author had the state copyright, which lasted forever as long as the work remained unpublished. Once the work was published, then the federal copyright took over. In 1978, the Copyright Act of 1976 went into effect. That act did away with the state copyright, and made the copyright effective when the author fixes an original work in a tangible medium. Further changes in 1989 to align US Copyright law to the Berne convention did away with the need to put a copyright notice on the work. What does the "poor man's copyright" do for you? Not much, really. If you need to prove that the work is yours on a certain date, then you would be doing so for an infringement action. If you bring an infringement action, you need to have your work registered with the copyright office. The "poor man's copyright" really grants you no protections under US law.