It might seem obvious, but one key to developing a successful freelance career is applying for jobs. Constantly. Filling out applications, writing cover letters, and submitting bids for projects can be time-intensive and incredibly boring, but it really is the only way to get your career started.
When I first started freelancing, I spent hours each day looking through job postings and applying for anything and everything that seemed like it might fit my skills. While my goal was to work full-time as a translator, I also occasionally applied for copywriting and editing jobs as a way to supplement my income. I ended up getting a couple of long-term flexible copywriting jobs that weren't particularly interesting, but they were incredibly important because I always knew that if I didn't have any translation work, I could rely on them for income.
Over the past year and a half, I've done translation jobs of every type and size, from 200-word marketing translations to an ebook on web programming. Some clients have only hired me once, others have contacted me a second or third time, and a handful have become regular clients who send me work frequently. As a freelancer, the goal is obviously to have a group of regular clients who keep you busy full-time, but I think that's something that takes several years to attain.
While there are some weeks that work from regular and repeat clients manages to keep me busy, it's still incredibly important that I keep applying for jobs. However, now that I have more experience and know that I have some clients I can rely on to send work, I can be a bit more discerning. A year ago I was a bit more flexible in terms of rates, but now I have a better understanding of the value of my work, so I'm more confident in sticking to my standard rates, even if it sometimes means not getting a job because the budget is too low.
In any case, my advice to new freelancers out there is to apply for every job that a) fits your skills b) meets your payment expectations and c) you feel confident you can complete to a high standard. It may be monotonous constantly filling out job applications, writing professional cover letters, and emailing résumés to prospective clients, but it's the only way to grow your client base. It's easy to become complacent once you've found a few regular clients, but it's important to remember that you can't count on them to always send you work, since it's only natural in freelancing that clients will come and go over time.