Firstly, Get the piece of art you wish to copyright.(THIS ALSO WORKS WITH LYRICS, POEMS, STORIES, LOGOS etc)  The art must not have been somthing that has been reproduced by yourself ie - a copy of someone elses work.  I recommend scanning the work, or photocopying it, so you can keep the original.  After you have a copy of it, buy an envelope which it will fit in, and take it to the post office.
The Post Office is an official government building, which is one of the main reasons this works.
Address it to yourself, and ask the person whom is serving you to DATESTAMP IT.
This is a vitial part in this process, as by date stamping the envelope shows that the art inside was copyrighted on that day.
Once you recieve the artwork through the door, DO NOT OPEN IT.
Thats it fully copyrighted, and if ever someone was to steal your art, you have proof that it was yours on that date stamped on the envelope.  If it was ever required to prove that the art was yours, in law or otherwise, you would show the envelope, and whats inside.

I send 3 copies of each of my art just incase.  Dont know why, but i feel more secure with three copies in three seperate envelopes.