There's a world of opinion on how to promote yourself, design and implement your website, and even what to wear in the office. (Some people advocate dressing up, which creates a better attitude for some and probably little to no change for many.) And, above you, you have to network -- make all those connections.
Many of these tips are good, which is to say that they help some people and don't necessarily cause harm. But in my long experience doing freelance work, there is something forgotten by many that makes a major difference to your business.
Get better. Get really, really good at what you do.
I've seen many writers, designers, programmers, consultants, and the like take umbrage when criticized by a client. It's not easy and clients are often wrong. Unfortunately, they're often right.
The most certain way to improve your business is to become out standing in your field. Work harder. Be more professional. Dig deeper in research. Push the limits of your creativity. Reduce headaches for clients. (That is an enormous one.) Avoid being a prima donna. (Another biggie.)
When you start going past what the typical freelancer in your field does, your business opens up. You start developing business relationships with clients that last many years. People think of you when they have assignments because they know you'll turn in good work and you won't be annoying in the process. When someone moves from one company to another, there's a good chance they'll take your contact info with them.
It's hard to over emphasize just how rare it is to find good freelancers. I remember taking on some editing assignments for a couple of different clients some years back and being absolutely dumbfounded at how many writers acted, for example. Lazy, complaining about having to rework things that weren't up to snuff, late, making excuses. My sympathy for editors and corporate clients jumped. If this is what they dealt with regularly, I'm surprised they could keep their sanity.
I've heard similar things from successful freelancers over the years. Clients say that it is basically a relief to work with them. The more clients can count both on the quality of work and know that if there are problems they can get them resolved, the more they want to stick with the people who can deliver. This is why good freelancers can become entrenched at clients and make it difficult for others to break in. The clients simply don't want to take another chance if they don't have to.
Your single biggest marketing advantages are doing good work and being easy to deal with. The combination isn't a magic wand. You'll have clients who want something different than you can provide, or who are so difficult that the fees you can get no longer justify the time you spend. But, on the whole, be really good and your business will soar.
To get good, you have to be honest with yourself. What are your shortcomings? Have you settled for trite ideas? Do you make the extra effort to bring something substantial to an assignment? How often do you examine your work and look for ways to make it better? Do you regularly look for ways to lessen the strain that clients feel all the time?
Everyone can improve. Not everyone cares if they do or wants to. Those who do will find a lot more success.