If you’re in the services business, you should subscribe to the Wellesley Hills Group newsletter. In their most recent article about "Staying Top of Mind" with clients, they found that most service companies are scared of "pestering" their clients with things like newsletters and personal notes. On the contrary, most clients appreciate hearing from their vendors every once in a while (just don’t overdo it, and keep the sleazy-sales tactics to a minimum).
Here’s the article: http://www.whillsgroup.com/pages/25_john_doerr_principal.cfm
The big takeaway? Stay in touch, by occasionally telling clients what kind of projects you’re working on, or recently completed. They usually react with, "Oh cool, I never knew they did that too." And you should mix it up—send hand-written notes, articles, personal emails, and email newsletters (ahem, we recommend MailChimp for that last one).
If you don’t keep in touch with your customers, they forget you exist, and hand their business to another vendor.