I had to juggle a staff of over one hundred writers, photographers and monitor an annual budget of seven magazines and a weekly newspaper. Writers that I previously worked with wanted an agent to represent their work. There are numerous photo agencies that represent photographers and most writers don't have the time to market themselves. Send a letter explaining what your area of expertise is. Most agencies are very selective about the type of material they handle.
Stories have to be unique and very well written.
Avoid sending travel articles to agencies, since all papers and magazines are already inundated with travel. We, in particular, look for stories that an international audience can relate to or personalities that very rarely speak to the media. Don't send sloppy samples or copy with spelling mistakes. Treat an agent as you would treat an editor. Don't harass an agent since their time is very limited.