You gain both knowledge, and relationships with people who have similar interests. It isn't necessary to get a degree in the subject, but any class is sure to help. When one is near you, be sure to attend. You should also write people in the field and solicit their advice or stump for an interview. The net is especially good for this.
People are more apt to reply to e-mail than a formal letter, because of it's casual nature.
The key is to get something under your belt. Try to come up with magazine article ideas and pitch them to the magazines you subscribe to. Find more general articles that may be of interest outside the industry. The important thing, no matter what your goal, is to have a writing sample you can show to prospective clients. Maintaining expertise in a field is an ongoing process.