Can I confess something to you? Nothing annoys me more than the "poor man's copyright" myth.
The "poor man's copyright" myth is based on the idea that one could simply print out your artwork on some copy paper or put it on a disc, date it, put it in an envelope, seal it up and mail it to yourself, and that in a court of law the date on the envelope, stamped on by the US postal service, will hold up as evidence of when the piece was created, thereby giving an indisputable copyright date.
Hah. Nice theory.
In reality, unsealed, empty envelopes make it through the mail system on a regular basis. You could easily put a batch of envelopes in the mail, addressed and stamped, but unsealed, and watch them all come back to you. I have had this happen numerous times with bills and both business and personal correspondence. It just doesn't work.
In fact, judges have thrown this out in the past. It's just not proof beyond a doubt. Sorry!
You can use certified mail, since that envelope has to be sealed before it can be mailed, but what do you actually get from a poor man's copyright? Not much.
With "poor man's copyright" you can sue for actual damages, which are extremely hard to prove. You might have a copy of the offender's books and be able to say, "look, he made $9,000 from my art!" but then he can say, "yes, but I spent $8,999 worth of my time and expertise marketing that artwork so that I could make a $1 profit." Ouch.
So if you're really concerned about your artwork, the best way to go is to register it with the copyright office. It involves paperwork and a little money, but in the same situation you can sue for punitive damages. Punitive damages are exactly what they sound like; Punishment. You can get punitive damages that exceed what they actually made off of your artwork. In fact, if I remember correctly, you can sue for up to $150,000 in punitive damages for the illegal use of 1 artwork.
It's important to keep in mind that lawsuits like these frequently put small business owners out of business. Cases like the ones mentioned above can stretch on for years and cost hundreds of thousands of dollars. If you win, you can sometimes get your legal fees paid for by the offender as well, but the fact of the matter is that very few people who are stealing artwork actually have that kind of money laying around, or even have enough assets to cover that kind of money. You may never actually see any of it.
In most cases a strongly-worded cease and desist letter will convince the offender to cut it out, but I have been told, and I am not sure if this is true, that if you send a C&D, the recipient can send a letter back that says "okay, sue me." and then you HAVE to sue them. So you should definitely consult an attorney, or at least someone with more expertise than I have, before firing off C&D's in every direction.