I sign the back of most of my stuff (if there is room) with a sharpie or paint. I am still trying to figure a way to sign a piece of jewelry...
I also recently started signing and dating the front of portraits in pencil before I spray finish them. The pencil is light enough to not distract from the art, but still there.
When I frame a piece I cover the back in craft paper, it hides any less then perfect edges on the back side. If the project is for someone in particular I write a secret message to them, otherwise I just write something fun, then hide it with the craft paper.