The update e-mail is sent when I have something new happen to my business. It does not need to be big or exciting, just something that's either changed, or improved. When something like this happens, such as an update to your website, a good change in business policy, just about anything that could be viewed as a positive thing, send out an e-mail describing it. When I update my website, I like to send out a broadcast e-mail to all of the people that I am currently working with, or have worked with before. This reminds clients that you are working hard, and motivates them to view the new update. Although this is not direct marketing, it does start the gears turning. Almost every time I have sent out an update, a client has contacted me with either more work, or desire to purchase an image.
My most recent update described my cover and spread in Photoshop User magazine this month. Next time you have an update, send out that broadcast e-mail to everyone, and enjoy the work that may come from it.