Participating in professional associations can increase your networking base. Get active in the organizations to which you belong. Join or chair committees. Your willingness to lead and get involved may easily translate to new business contacts that turn into leads for projects. You will also benefit from attending their periodic meetings; they will help keep your skills up-to-date and will help you meet new people (read potential clients).
Additionally, many organizations have publications that publish timely and relevant articles. You should write articles yourself. Doing so will definitely increase your visibility and position you as an expert in the area. Many organizations have member directories and separate directories for consultants. Maintaining listings in these publications is an inexpensive source of advertising throughout the year and may well bring your business.
The organizations may also have a "Consultant Directory" in which you can advertiser your services. This is an inexpensive ways of advertising and well worth the investment. If you are going to specialize in developing solutions for a particular industry, subscribe to trade journals and join associations for that group as well.