From the [non]billable hour:
The single best marketing strategy in the world is to find your best clients and ask them, "How do I get more clients like you?"
Involve your best customers in helping you find new customers ("We'll send a free bottle of our wine to someone you recommend"). Let your best customers know that you know who they are, and are glad to have them, and respect their opinions.
I'll never forget the free coffee mug that Amazon sent me one year (back when they were young and struggling), just because I ordered a lot of books that year.