Have you tried offering your current clients a "thank-you" discount if they refer a new client to you? (Only if the new client books and pays).
A lot of the pet places near us (like Pet Supplies Plus) have a community bulletin board that you can post business cards. Design a website with the basics on it (your contact info, some happy pics of you with dogs), with your info (email, phone, rates, discounts, etc.), and have that on your card.
You could try posting add regularly on Craigs LIst -- those are free.
You could try marketing yourself as a less expensive (if that is the case) and more personal service than the traditional kennel, without the same risks of disease that comes with using a boarding kennel.