Even if you're automatically granted copyright under the law, it's a good idea to officially register with the Copyright Office so you can protect your rights more explicitly. Another thing people will do sometimes is put their copyrighted work into a mail package, mail it to themselves, and keep it stored away somewhere, unopened. The mailing date acts as a bounding point so you can establish that you created something first, if need be.
This is more or less an urban legend, which might have had some merit in pre- Berne Convention days, but is just a waste of postage now. The only official registrar of copyrights in the USA is the Copyright Office (part of the Library of Congress) and a postmark isn't as good as a notarized document or even just some timestamps on a server log as long as there's no good reason to believe they've been tampered with.