I've only been freelancing for about 5 months, but so far I've just been referred into work from my friends on Twitter. My first job, the client approached me through my site - ever since it has been followers on Twitter who need work done themselves, or who have recommended me to whoever they know that needs a developer.
I think this has probably been helped to some extent by networking with a lot of these people in person at meetups. There's no substitute for coming across as a nice guy who is knowledgeable and enthusiastic about what he does.
Letting people know is as simple as putting it in your introduction when you first meet. "So what do you do again?" "I'm a freelance web developer actually, at the moment I'm working on X but I do a lot of different stuff including Y and Z." There's no need to go overboard on 'selling yourself', if people like you and know what you do that's enough.
I think this has probably been helped to some extent by networking with a lot of these people in person at meetups. There's no substitute for coming across as a nice guy who is knowledgeable and enthusiastic about what he does.