Work those referrals
Once you get established as a freelance writer it is pretty likely that you'll get more opportunities for work through existing clients. I’ve gotten some of my other work through those first jobs I got. In fact, most of the clients I've had over the past four and a half years have been people I contacted for a story and they later called me to inquire about my interest in freelancing for them.
These clients are wonderful for several reasons -- they come to you, first of all (and best of all) and they are well familiar with your work. You have already sold yourself to them without even meaning to, and they want you! Consider all requests seriously and don't brush any aside, even if you think you already have enough work. They may be able to make it worth your while, or even worth it for you to let something else go.
Follow up on those referrals!