Upload & Sell: Off
This is what I do, and what suits my subject matter, papers and presentation. YMMV and nearly all my images are full color natural history. In general, I have 3 formats in which prints are received by a client: framed and matted papers, papers trimmed to edge of prints without any framing, and gallery wrapped canvas.
Framed papers: I sign the print itself and the matte. The paper is signed in the margins that are not printed on and hidden by the mat when framed. This margin is typically ~2" of white space that I leave on the print and is behind the mat. This would only be seen by the customer if they removed the print from the frame. It just serves the purpose of keeping your brand on the print longer than if only signing the mat. For instance, if the customer decides to re-frame the print down the road, the print will still have my signature. Customer could trim off this margin and remove my claim to the photograph, but I understand that trade-off and believe this strikes the balance I prefer. On the mat, I sign bottom right with the year. Title of the image goes bottom left. I used to number my prints with an edition, but no longer do this.
Unframed papers: I deliver these in an archival sleeve, containing the trimmed print (no margins) and an unmounted archival quality matboard of the same size (only for rigidity). These prints I sign the back of the print with signiature and year bottom right.
Wrapped canvas: I sign the canvas bottom right with year. I don't include a title on canvases.
Choice of pens/markers/pencil, etc. is one I spent some time choosing. In all cases, I use ink that is archival certified. On papers and framed mats, I use Sakura Micron in black and the .01 size. On canvas, I will use the Sakura Micron or this Pilot pen in Silver if the background is too dark. Sometimes the silver is also not appropriate, in which case I'll use one of several archival white pens I have. Its tough to find a good white or silver pen that is archival, but here is one that is pigment based. The Sharpie Poster Marker in white is supposedly archival as well, but I haven't used it. Maybe the idea of a "Sharpie" is what's turning me off.
Lots of ways to do it, but this is what works for me. At times I have done less in the way of signing and actually received feedback from buyers that they wish the signature or other details were more prominent. Its all a balance I believe and there's no right answer. Its all a part of your work or art if you think of it that way. Its interesting to see the way others have adapted the process. I'd love to see some pics of examples if anyone has any. I'll try to come back and post some of mine.