The most basic content provider advertising is placing an advertisement at a high traffic freelance writers' site. But probably the most effective one is using keyword phrases to attract traffic via search engines of prospective clients or site owners looking for content providers. This targeted traffic would then be directed to a simple web site or blog where you can then showcase your online writing services.
Whichever method you choose for advertising your content provider services, it is important that you focus on the needs of the clients that you are targeting