I've always been a big proponent of staying in touch with clients. Not only does it allow you to see the results of your actions -- how many times have clients not gotten back to you to report how their job search is going? -- but it also fosters repeat business and referrals.
E-mail is an easy inexpensive way to keep in touch (I recommend using an e-mail newsletter for consistency), but at least once a year, you should also mail something to your entire client base. It can be a special offer (perhaps a postcard each September to coincide with Update Your Resume Month), or a holiday card, or even a print newsletter.
In the next week, I'm mailing out more than 500 postcards to promote the release of our first Special Report, "Making Money as a Resume Subcontractor." While 500 names is only a fraction of my mailing list, I know there will be several dozen people who will have received e-mails from me but didn't open them (or their spam filter caught them) who will be surprised that the Resume Writers' Digest newsletter is back.
By the way, if you want to order the "Making Money as a Resume Subcontractor" 30-page special report (available for PDF download), you can do so here:
Mailings have an advantage over e-mail -- they're hard to ignore. And postcards are even better, because you don't have to open them up. So plan a mailing and take advantage of the power of print -- to reconnect with your clients, to spark new business, and to generate referrals.