As a former COO of the leading European design agency I am familiar with this topic. We started super-small and then become one of the most influential companies of its kind. All in 10 years.

What has been mentioned in the answers above is true. Testimonials and Referrals from happy clients are always the key and you should have them no matter what.

Although, the tools and channels I have seen working the best along with providing quality services were:

• Publishing extensive amount of work on social networks for designers
• Speaking on conferences about the latest trends
• Running very specific and targeted online campaigns via AdWords and Facebook.
• Winning awards for the work

If you would like to know more, let me know.

Regards, Filip