Contact local newspapers to solicit an article on your play. They may also provide some form of free advertising if you inquire into the possibility. This is great exposure, and newspapers are often looking for community events and undertakings to write about. Don't just stay in your town; hit neighboring communities' papers as well.
Contact area radio stations. As with newspapers, they are always seeking out local events. An interview on the radio can bring a lot of attention to your play. If the station cannot arrange an interview, they may offer a public-service-announcement type of promotion, which is usually done at no charge.
Contact local television stations. They, too, are always looking for segment ideas. Contact all the stations in your area, not just the ones you watch.
Hang up fliers everywhere you go. Post offices, grocery stores, laundromats and libraries usually have bulletin boards.
Send out an email to a listgroup devoted to theater. Yahoo and AOL have many groups for these purposes. Find one in your area to network with.
Send fliers home with the students enrolled at local schools. This is a great way to hit hundreds of homes at once, at virtually no cost. Be sure to first get the approval of the principal or secretary at each school.
Post an ad on Craiglist. One of the hottest new tools around for advertising and networking, and it is free!