This article is by Clint Watson, former art gallery owner/director/salesperson and founder of FineArtViews. You should follow Clint on Twitter here.
I posted this as a comment on the Reddot Blog and expanded it a bit here.
One marketing idea you can utilize is to contact your customers periodically to provide cleaning and maintenance on past pieces that they've purchased from you. This can often lead to another sales opportunity, but you don't have to make it into one, you can let that happen naturally. This is also a great technique for those who are unsure or shy about being too "salesy."
Back when I owned an art gallery, I used to do a similar thing with great results: Each time someone purchased a painting I would make a note to call them back in six months. At that time, I would let them know that their painting needed to be brought in (or picked up by me, if feasible) for the final varnish.
This re-connection often led to a series of events that ended in another art sale.
It's also a great thing to do because it's not solely for marketing purposes, although that's a fantastic side-effect.
In fact, the final varnish is critical for oil paintings and often can't even be applied for six months after the painting is completed (or even a year in some cases). In today's fast-paced world where everyone needs results and sales today, most artists can't afford to wait the six months, let alone a year, to apply the final varnish and then ship the painting to the gallery. So lots of paintings end up being sold without the final varnish. (Note: the final varnish is NOT the temporary liquin retouch varnish that CAN be applied immediately to give an oil painting that glossy, varnished look - that will wear off). [Update: I originally mis-typed and said "liquin" - that is not what I meant, do not varnish your work with liquin (see the comments below for reasons why), I had originally meant the spray retouch varnish and my years away from the gallery business had muddled the two in my mind - apologies.]
I may be telling you something you already know, but a quick story: The varnish on an oil painting serves the same purpose as the glass on a watercolor - protection. I became a full believer in this when one of our customers had a house fire which luckily was put out before the house burned to the ground, but it did cause a lot of smoke-damage to their extensive collection of oil paintings. Out of desperation, they brought the paintings to the gallery - I would guess around 50 oil paintings - and the pieces looked like someone had framed 50 black canvases.
I didn't have high hopes.
We took the art to a local art restoration expert. And amazingly, she was able to completely save every single painting. And she told us why: A few months earlier we had contacted this particular couple and offered to varnish all of their oil paintings. The restoration expert told us that the varnish had saved the paintings. All she had to do was strip the varnish off and the paintings themselves, perfectly protected, were in pristine condition. All we had to do was apply a new coat of varnish.