I didn't sign my art until last year... as I saw it, my art wasn't good enough for someone to steal, or at least I'm not well known enough for someone to steal it. For things I made as adoptables I'd make a symbol sign that was partially transperent so it didn't detract from the images, but other then that, no signage.

Last year I started signing things with my real niame so I could sell them, as most of my work were real life monetarily paid commisions, and I beleived it was time to take pride in my work. I sign small, along the edges of the bottom of my artwork, though clearly enough you know it's there, and a bit of a bi-atch to remove. If I displayed the image elsewhere on the net when it was a commisioned image for someone else, I put a big watermark on it, as that person specificly bought a peice of work from me for THEM, not for everyone else to nab for their profiles.

Luckily most of my work is still unknown, so I havne't had any known stealing yet... but it's best to be cautious.