A while ago I talked about using one of the freelance websites such as Guru or elance to market your writing skills. These websites are job posting boards that allow freelancers to set up profiles and screen jobs. They provide a certain amount of security to freelancers and help new ones find employees easier.
However, there are other ways to freelance. Freelancing outside of a freelance website is more difficult in some ways because you will need to be able to maintain a positive attitude about your work despite many rejections. It also is a little more difficult to find clients for your work.
Most news magazines and newspapers use a certain amount of freelanced articles. If you are interested in doing this, be sure to research the guidelines for each agency, buy a copy of the AP style guidelines (note this is not the APA styles guidelines) and follow them in addition to the agency's guidelines, and develop tough skin. These same agencies may also have blogs or online content that they use freelancers to write.