FINDING VALUE IN THE "Poor Man's Copyright" PRACTICE
If your work is anything like Robert Therrien's Under the Table, you probably shouldn't spend the money to mail a copy of it to yourself. Otherwise, however, it's not the worst thing you could do.
I recently heard a pop theory that if you don't want to spend the money to register a work with the Copyright Office, you can get the same or similar protections by simply mailing a copy of it to yourself. The practice has been dubbed "Poor Man's Copyright," and as best I can tell, is based on the theory either that the postal service automatically confers heightened legal significance on the works it carries, or else that the postmark on the envelope or package will help establish a date of creation, which could be legally significant.
Unfortunately, but hopefully not surprisingly, the first explanation is simply wrong and has no basis in the law. If you're considering mailing your work to yourself for that reason, therefore, I'd encourage you to save yourself the postage (which could be significant if you're, say, a sculptor!). The second explanation has some merit that previous online commentators seem to have overlooked, however. While I don't dispute their conclusion that the theory has never been dispositive or even highly valued in any published court opinions, I see at least a potential value for it that prevents me from dismissing it outright.
The question of whether your work existed as of a given date can certainly be significant in a copyright infringement action. For example, if someone claims your work infringes theirs, establishing that your work already existed at the time theirs was created would be extremely helpful to your case. A postmark on a sealed envelope containing your work isn't the worst way I can think of to prove that your work existed as of the date listed. It may not be the only way, or even the easiest way. For that, I’d suggest e-mailing the work (or a picture of the work) to yourself. The metadata associated with that email will be at least as valuable as evidence as a postmark and doesn’t require the same kind of special handling to maintain its integrity. But the postmark isn’t meaningless.
Admittedly, the evidentiary value of the postmark declines where you're trying to establish that someone else infringed your work. That's because proving your case in that regard requires not only that your work existed before the defendant's, but that the defendant accessed it prior to creating his or her own work (among other requirements). Since whatever evidence you produce for the "access" element will likely also establish that your work existed first (e.g. an upload date on SoundCloud or YouTube), and since the postmark itself does nothing to show access, the postmark evidence will be superfluous in this situation.
Of course, actually registering a work with the Copyright Office does more than just establish your work existed as of a given date (although it does that too). In fact, a timely copyright registration can entitle you to heightened damages if someone infringes your work, and registration is a pre-requisite to filing an infringement lawsuit in the first place, which means you’ll have to formally register your work to bring an infringement action down the road anyway. Poor man’s copyright should really only be on the table, therefore, if the copyright registration filing fees (currently $65 for a single work) are prohibitive for you at the moment and/or if the likelihood of infringement is very low and you’re comfortable with the gamble. If that’s not the case, while there might be some merit in the poor man’s copyright practice, it probably isn’t for you.