It takes being vigilant and being willing and able to take legal action. YouTube uploaders always hated the fact that Warner was so militant about take down orders on that site. Otherwise, small size and watermarks are your best bet.
I use signatures on a separate piece of paper which I put right beside or slightly covering the art, so that it's hard to edit out. Also if it's personal art I'll put a DA watermark over it too, because I don't need to post the unwatermarked version of my personal art seeing as I have the art myself physically.
The best way to protect your work is just to use the existing copyright law. As long as you can provide the works and a set date. Your work is automatically protected under copyright law. You can get more info on the subject here.