I do. Some don't.

For me, I sign the painting after the final coat of varnish has completely dried. I find a clean spot along the bottom of the painting and slowly and clearly sign my name. Using a permanent "marker," either in black or white, I legibly sign my name where it is easily found. Don't hide your signature - that is called  marketing.

Watch the video for my rant! 

Some artists sign their work with paint, using a paint brush. That has never worked for me - my signature looks illegible. I get the best results with a permanent pen or marker. My favorites (right now) are  Faber Castell Pitt Artist Pens or an  oil-based Sharpie Paint Pen.This one is available in black, white, silver and gold. Both the Faber Castell and Sharpie Pens have a variety of nib sizes. Again, both are permanent - other signing pens are too, but these are the ones that work for me.

Some of you asked about signing multiples - such as diptychs, triptychs and polyptychs. Thank you! I sign on  ONE PANEL only - and on the back of each panel I sign, date, state the series title and designate with a diagram, the placement of where it fits in the overall painting. That way, your customer will know how to hang it! Clever, eh?

PS - the signed date should appear on the back of all paintings. I do not put the date of completion on the front. It is not necessary to put a © copyright mark on the front.