Building a membership can be done in various ways. My background is in not-for-profit television. As the membership manager, we sent membership letters out from magazine address lists we purchased for that purpose. Certain demographics can be targeted fairly easily in this way. In working for public television, volunteers were a HUGE part of our on-air membership drives and live events. We drew on-air talent, telephone bank operators and even studio camera personnel from our volunteer base. To fill telephone banks, we would approach clubs, groups, or businesses to man the phone bank for a shift. In return for this service, we gave them an on-air thank you. This reached many people. Maybe current members could gather a group in their area to man a public t.v. phone bank, print up an NPCA banner to hang on the front of the phone bank desk, and maybe even prepare a little spiel in case the talent asks about our organization. All it costs is time and printing a banner (unless we already have one.)
In addition, we could create a few :30 spots for Public Service Announcements on various radio stations. I believe these are still free to non-profits.
A little ad or two well-placed in industry magazines and on Pilot Car websites couldn't hurt, either.