Your best clients are your existing and past clients. This powerful statement has been true for the past hundred years. A common mistake entrepreneurs make is that they spend too much time, energy and resources trying to attract new business.
Developing a relationship with your current clients and they become advocates for your services, and can recommend and refer you to other prospective clients. Keeping in touch with phone calls, informative emails can multiply our referrals.
If you want referrals ask - "I'd appreciate your referrals for my services", "Is there anyone else you know that I could contact that could use my services?" - Ask simply and directly. In your email blasts make sure you have the "forward to friends" option selected. Put a message in your template - "I'd appreciate your forwarding this email to anyone you think might be interested in my work". You can also clearly state "I am looking to expand my business, if you know other {photo editors, art directors, curators}, we'd love to hear from them."
Last tip: join www.linkedin.com and research your past clients, friends and colleagues - this is a great way to network in a professional manner and build your network - and ask for a recommendation.