Be confident. From my experience, you need to master the art of selling yourself, along with knowing your skills and strengths and how much they're worth (i.e day/hourly rate).
Also, act like a business rather than a 'freelancer'. For example, when quoting a project, ask for a percentage of the total costs upfront — this generally weeds outs the dubious clients and sets the tone that you're not some hack. It's become a bit of the wild west out there with some of the race-to-the-bottom freelancing websites, so good luck!
