As a freelancer, your career is in your hands. So is your career development, which means you should be continually learning new skills to attract new clients and better serve the ones you already have.
For example, until last year I had only worked with traditional publishers. When I decided to publish a POD book, I spent several weeks learning about its pros and cons, and evaluating POD companies (I eventually went with CreateSpace). Now I can advise ghostwriting and coauthoring clients about the advantages and drawbacks of POD and traditional publishing, as well as e-books. That helps set me apart from other ghosts and makes me more valuable to clients.
Fore more on expanding your skills, check out Susan Johnston's excellent post at ebyline on 5 skills that make freelancers more marketable.
And consider what tools you should add to your freelancing toolkit. As the publishing industry changes, your clients' needs will change, too, and you must be able to address and meet them.