Most of my paid or otherwise-supported work comes as a result of inbound requests based on a) prior work that doesn't suck or b) word-of-mouth driven by kind people who have a positive impression of me.
When I look back at my editorial relationships -- from The Somerville News and Dig Boston to Fast Company -- they can nearly all be traced back to a) or b) above. So in thinking about self-promotion for writers, I would think about how to drive one of those two strategies. Some ideas with headings:
Produce As Much Work That Doesn't Suck As Possible and Make It Available.
Years ago I met comedian and actor Jeff Garlin. I asked his advice on "making it" in comedy, and his answer was, essentially, "focus on doing good work, and opportunities emerge from there."
From a more practical perspective, just produce a ton of work that's interesting to you, and make it as good as you can.
Encourage Good People To Learn About You And Your Work
Engage with the community of people and ideas around the people and ideas that interest you.
In the digital sense, this may mean following certain accounts on Twitter, reading and commenting on relevant blogs and websites, or reblogging the hell out of animated gifs on Tumblr.
In the physical world, this could mean showing up at panels, conferences, happy hours and other social-professional gatherings, having actual out-loud, in-person conversations with other humans you know in the same areas of interest as yours.
Ask People Who Already Like You For Their Help
Choose a few supportive people you've worked with or had a personal relationship with. Send them personal notes explaining your current location and work, and that you are looking to connect to the community where you are currently living/working. If you find 5 to 10 people to reach out to, odds are high that many of them know someone or are a step removed from someone who could get you closer to your goals.
Do Unto Others....
Keep in mind also that a certain way to improve the impact of your own self promotion is to promote others. When you hear about an opportunity, consider who you know who'd be good at it, and hook that person up. When you have positive reactions to others' work, don't keep it to yourself. Tell people! Be the change you want to see!
To Sum It All Up
I'm ultimately promoting a simple formula: produce good work + tell people about the work you do + be active in the community of people doing similar or related work.
With this basic approach, what I've found is that folks start to learn about what you are about and good at, and they may seek your input, participation, or labor.