This. My favorite thing to do when I watermark is actually put several in the piece itself. A big one in the middle, overlayed, 50% fill (Photoshop speak wooo), and then one or two others in the corners, one at least at 70% and the other at somewhere around 30 or 60% depending how I feel.

If they're that desperate to snatch your work, they have a lot of watermarks to go through, and just end up giving up.