First you make a facebook fanpage and increase 500-1000 likes then share your all writing services.I am sure you will get revenue.
I always like to suggest ppc advertising because it's the quickest way to get eyes on your site. Also forums like this one and warrior forum are great for exposure. These two really have the targeted traffic you're looking for, so I would advertise in their services section.
Some of the best ways i could see promoting a writing service is by Social Media (Twitter, Facebook) Video Marketing Seo (Very Time Consuming) Article Marketing Banner Ads Word Of Mouth Warrior Forum Press Release
To promote writing service, your articles have good content to be approved on articles sites. After completing for sumitting, you should apply SEO service to promote them such as forum posting, blog comment, social bookmark, etc.
Look and see if there is a writing section here.. over in the warrior forum there is a warriors for hire section..
of course SEO will help you get the goal. also you can try to use facebook and twitter to post link or pictures to attract people. It is useful if you post a video on them.