There have been several notable copyright court cases in which the original artist/author (usually authors) has successfully proven their copyright without having filed an official form at the copyright office. This was done by mailing the original documents, including notes and scribbles, to one's self, allowing the postmark to become the date of copyright.
Wondering how you can legally do this with your digital art? Whilst in the creation portion, save work-in-progress (WIP) files. ON THE DAY YOU FINISH YOUR PIECE, burn each of your WIP files, the finished program file (.PSD for Photoshop, .UFR for Ultra Fractal 4, etc...), and any end-res