My comment on: Promoting Your Services (Link broken so I removed it, original post from PoeWar, very good site on writing and resource for writers)
I think another great way to promote your business is to write and post to your own site. I encourage management consultants (the people I most deal with - example: Peter Scholtes) to create an online presence that illustrates what value they offer. And for writers it should be an even better fit (you can show exactly what you offer - your writing). Setting up a blog is a very easy way to do this. You can use Wordpress to set up a free blog without any technical knowledge.
Then you can market by doing what you want to do: writing. You will have to take some other steps to market yourself also but a home on the web highlighting your writing is an excellent tool.
This next tip is probably most suited to someone getting started as a professional writer, or one just getting started online, but really can work for anyone. Basically unless you are so busy with paying work that you can't fit it in, I think it is a good idea (and at that point continue your blog by linking to the paying work).
I also think writing guest posts for other blogs could be a useful promotion strategy. You will increase those that know about you and gain links back to your site which will help your site grow. And, another benefit, for writers is you get to practice. You can experiment, and try new things. While I don't think of myself as a writer, I have written a great deal building my web sites. The sheer act of writing for different audiences as a guest blogger I think would be good experience for a writers.
I tend to favor thinking about marketing differently - especially for small businesses. Amazon spends very little advertising. Instead they see free shipping as marketing. They see paying referral fees to websites that provide them paying customers as marketing. Very smart, in my opinion. So for writers think of writing posts for your site and for some others as a form of advertising. Plenty of very successful people market using their free writing online: (example: Neil Gaiman, Joel Spolsky...). And some turn that free writing into a platform to sell ads and then make a living just doing that: Dooce.
Another good example of innovative marketing: Giving Away Your Service for Free on Weekends (from Joel Spolsky mentioned above).
Related: My web home - John Hunter - Seth Godin on Marketing and the Internet - Your Online Identity - Innovative Marketing Podcast - Don't Lock Your Content Inside a Proprietary System