Smart Company has run a networking advice column for people who are self employed. The main gist of it is "If you work alone, make special efforts to connect with others."
As a writer who's worked freelance for a few years, I can certainly relate to the feeling that when working alone you can lack contact with your peers which can help you develop in the job and network successfullly. This article has a few useful tips on how to build or maintain relationships.
It recommends a few American professional organisations to join for networking and professional development. If you're a member of a professional organisation, let us know in the comments section what group you're a member of, and what you get out of it.
Relationships for the self-employed [Fast Company]