The other day I was talking to an artist in Edmonton. He is the one who in 1980 taught me to keep track of my paintings with card files. Here is a photo of one:
Using 5x8 inch cards, I take a photo of every painting and staple it to a card. I number my paintings by year, month, and then 1,2,3 and so on, and I put that number in the upper right corner.
The name of the painting goes on next. Sometimes it is a temporary working title, which can be changed later. I put the size of the canvas, and the dimensions, and the medium used (i.e. acrylic, or oil, or whatever it is).
If I have varnished the painting, I jot that down. When I sign the painting, I note on the card where the painting is signed and whether I have used initials or my whole name. On the back of the card, I record when I send it out on consignment, what gallery it went to and when. If it is sold and I know who bought it, I note their name.
It sounds complicated, but it has worked well for me. I have sent off many paintings over the years; the galleries aren't always good at documenting the ins and outs of inventory, so I make sure that I know.
Here is a link to the website of Douglas Haynes, who shared this piece of helpful advice with me:
http://www.dhaynes.com/