In an interview with Enrico Bertini, Moritz Stefaner, whose work you'll probably recognize, talks about his experiences as a freelancer and how he got started. Some of the highlights include how to get your name out there, important skills, and the demand for people who know data, visualization, and aesthetics. At nearly an hour long, there's a lot of good information in there.
The main takeaway: get started now, play with data, hone your skills, and the work will come.
Open Thread: What the **** is Visualization Anyways?
At the other end of the spectrum, what's the best way to find data viz freelancers?
and of course, you can always browse the FlowingData archives for people who do good work.
Good interview, have you done any visualization of click-steam data?