In addition to placing a notice on your work, it can be a good idea to register your copyright. This, too, offers you a way to prove that you own the copyright, if it should ever be challenged. In addition to helping you prove your ownership, registration of the copyright has a couple of benefits. While registration is not mandatory, you can't file a suit until you register it. However, you can register your work at any time within the life of the copyright.
Registration is a simple, and relatively inexpensive, process.
These forms are free from the Copyright Office. If you have an ad with both copy and art, use Form VA if the copyrightable material in the ad is primarily pictorial or graphic, or Form TX if it is mainly text. The registration is effective as of the date the Copyright Office receives everything from you. If it's a joint work, it lasts until 50 years after the last surviving author dies. For works created and published prior to 1978, the time period was much shorter.