Here are several ways to keep your name on your clients' radar screens without being annoying:
1. Consider sending out a monthly newsletter. My preference is one that visually promotes your distinctive brand (I like MyEmma.com because they will custom design your template to your specifications). Keep it short, sharing information that will be meaningful to your prospects and clients. If you do family portraits, share tips about what clothing photographs well against certain backgrounds, for casual or formal shots. Make sure your newsletter contains at least 2 or 3 shots of some of your newest work because it's not enough to talk about how creative or great you are; a fabulous picture can help you sell it, especially if you've introduced new concepts, lighting, etc. If nothing else you should be showcasing your expertise and uniqueness. And don't forget the professionally written copy, because great marcom copy writing captures the flavor of your studio (hip, traditional, flashy, etc.).
B.Send out a professionally designed and written direct mail piece about four or five times a year. If your client missed the email (or it accidentally landed in their junk folder) you'll be in touch without being too intrusive. It helps if you have a specific strategy, like a mailing tied to a certain time of year (holidays are great for annual family portraits, peak seasons for weddings, corporate photography, etc.) If you're not interested in discounting a specific thing to entice them to book with you (get 10 percent off a package, etc.), then consider offering a loss leader--book Package A or B by (give 60 day leeway) and receive a FREE 8x 10 (or whatever).
C. If you've won an award from your local chapter, partnered with another vendor, gotten editorial coverage in a newspaper or magazine, introduced a new service, all these happenings can be leveraged by communicating this info in a meaningful way via a simple e-mail to strategic clients and prospects.
As long as you are not bombarding them, which means you have to track who you've sent info to and when, keeping your communications methods varied while sharing interesting tips, advertising specials tied into key seasons, etc., you should be able to keep in touch in a smart and respectful way while feeding your sales engine nicely.
Best of luck!