This article is by Clint Watson, former art gallery owner/director/salesperson and founder of FineArtViews. You should follow Clint on Twitter here.
"Cube-grenade" creator, and back-of-business-card cartoonist Hugh MacLeod has built a career out of selling his art online.  Here's what Hugh says about his success and the role of email newsletters:
I’m a cartoonist. This is my tenth year blogging. My work has been mentioned in big media including Wall St. Journal, Financial Times, New York Times, and The Guardian. It’s been mentioned in bestselling books...on thousands of blogs, including many of the big, A-Lister ones...none of that “hot PR media action” has moved my business forward nearly as quickly or effectively as this one simple thing: My newsletter subscribers telling their friends about my newsletter, and suggesting that they sign up. [source]
Email Newsletters Work. Period.
Art marketing coaches, gurus, and experts encourage artists to utilize email newsletters - with good reason - they work.
We know email newsletters work, that's not even a question. In fact, I've even been known to say that I'm not surprised that your art isn't selling if your art is not being regularly featured in an email newsletter . At FASO and FineArtViews, we've seen artists be quite successful with email newsletters. They work well for us too -- you're probably reading this because you're a FineArtViews subscriber.
But How do You Avoid Sending Too Often?
Art bloggers often advise artists on the "right" frequency for sending an email newsletter. This advice generally warns artists not to send too many emails, you don't want to "overwhelm" your subscribers, after all. That's good advice. Here's the rub, though: the recommended frequency is all over the map. I've read recommendations ranging from once a month, to once a week, to no more than 2 times a week. Where are people getting these numbers? They seem to be based on gut feelings and personal preferences. If there was any data cited, I certainly missed it. Let's be a bit more strategic than that...
The .2% Solution
You can send your newsletter as often as you like...as long as you don't have more than 2 people unsubscribe for every 1,000 emails you send (.2%) . If you lose more than 2 people out of every 1,000 emails, then you're either sending too often, or you've strayed too far off topic (such as discussing politics in a newsletter where people were expecting art). I sure wish I could remember where I first read the 2% number cited, but I can't, so I did some quick research, and came across these statistics from a 2010 Direct Marketing Association research paper that reports - those who email to their own "house" list have an average unsubscribe rate of .77%. So let's say, based on that research, we'll set an upper bound of 7/1,000 unusbscribers .
What I'm recommending is this: for every 1,000 emails you send, you pick a number you can accept to lose...it can be as low as 2 or as high as 7 - but it can't be zero (zero is unrealistic once your list grows past a certain size). Incidently, FineArtViews shoots for .2% (our current unsubscribe rate is .04% (4 out of 10,000)).
How to Measure Your Unsubscribe Rate
First of all, you have to know who unsubscribed.  Yes, that means you cannot and should not send your newsletters with your regular email program using the BCC field for all your subscribers. Seriously. Don't. Do. That. What you need to use is a tool made for sending email newsletters like MailChimp, Constant Contact, or Aweber. If you're a FASO customer, we provide an email newsletter manager automatically integrated with your website (and Facebook!) that tracks all this stuff. Here's a screenshot of the FineArtViews newsletter stats inside a FASO account:
FASO's email newsletter analytics page.
The Perfect Sending Frequency
So what's the perfect sending frequency to ensure that you don't lose more than .2%? My answer is as often as possible. That's why we send FineArtViews daily. Send as often as you have information to share...just make sure it's personal, timely and relevant. If your unsubscribe rate is too high, dial back the frequency a bit until it's "just right"  In fact, you can even set up-front expectations for your subscribers by telling them what to expect on your newsletter signup page. Sending your newsletter every day is fine as long as the person knows what they are signing up for.
Don't guess - what works for one artist might not work for you. Send on the frequency that works for you and then track the numbers.
While you're deciding whether you should send more often or less often let me share with you a few success stories:
Remember Hugh MacLeod, the sucessful "cube grenade" cartoonist? He sends his newsletter daily. I look forward to them.
Here are a few other examples:
FineArtViews sends daily.
Groupon sends daily.
Facebook sends every single time there is an update.
Brian Kliewer's successful "100 paintings in 100 days" project sent daily.
Robert Genn (of The Painter's Keys) sends twice-weekly.
Linkedin sends their new updates daily.
Early to Rise sends daily.
The Daily Reckoning sends daily.
Thrillist sends daily.
You'll have to find the frequency that works best for you, anectodal evidence, at least, seems to suggest....err on the side of more frequent newsletters. But now you're armed with the .2% forumula, so you can find the perfect sending frequency for you.
Software Craftsman and Art Fanatic
PS - If you are afraid to send your newsletters more often because you think you'll be "bothering" people, consider this: "The only thing worse than being talked about is not being talked about" - Oscar Wild
 If you haven't read Hugh's, How To Be Creative, go do it now. He calls his cartoons cube grenades because they're designed to "start real conversations between people" - I believe it's a reference to breaking out of the groupthink that can take over in corporate cubicals.
 It doesn't have to be you sending the newsletter, it could be your gallery, spouse or agent. Just as long is somebody is doing it.
 If your list is small, say 50 subscribers, you apply the ".2% solution" to the total number of messages sent. So after you've sent 20 newsletters, you would have sent 1,000 emails (20 newsletters X 50 subscribers) - make sure you didn't lose more than 2 of the original subscribers, if you did, make adjustments to the frequency and the content.
 The word "unsubscribe" necesarily indicates that someone "subscribed." If you have a big list of your friends' email addresses in outlook, you don't have a "house" email list - don't spam them. You should (and legally can) send newsletters only to people who voluntarily sign up for them. That means you need a newsletter opt-in form on your website. If you have that big list of friends, I recommend you send each one an individual, personal email asking each person, one at a time, if they would like to receive your regular email newsletters. Each person who says yes - you can add them to your subscription list. Do the same thing with every single person who purchases your artwork as well.
 If you have a list but haven't used it in a long time, don't judge the unsubscribe (and bounce) numbers until you're a bit into your campaign. Lists get stale over time, so your unsubscribe rates will be higher at first...that's actually another reason it's better to send more often...people come to expect your newsletter. I once hit a very busy patch in life and stopped sending FineArtViews for a couple of months, people started writing me asking if I was OK, saying the newsletters had stopped and that they were worried that something terrible had happened...fortunately, we have since added several great contributors, so the show goes on even when I'm unproductive.
 More Direct Marketing Association Numbers:
Marketing emails using a house list are experiencing, on average:
- Open rates of almost 20%
- Click-through rates of 6.64%
- Conversion rates of 1.73%
- Bounce rates of 3.72%
- Unsubscribe rates of 0.77%.
Direct Marketing Association (2010) [source]