I don't reckon there's any one proper way to do it, but here's what I do and it seems to work OK:
I write the title and basic information about the image on the back, and sign my name below it. My basic information includes the camera and lens used to capture the image, the type of film (if it's a film image, and most of mine are), and the type of paper it's printed on, if the manufacturer doesn't imprint this on the back.
I use an ultra-fine-tip Sharpie to do this with since it's permanent but doesn't mar the paper. I can't imagine it has any adverse effects on the paper or the image on the other side, but then I print on thick papers.
