Advertising is hard. Here in New York buying ads is very expensive. The Folk Music Society of N.Y./N.Y. Pinewoods Folk Music Club prints hundreds of fliers to give out at related events & venues, & we ask our members to post fliers. My guess is that we get about one new person for every 1000 fliers, but this is just a guess. Word of mouth is best.
The on-line event listings are difficult, because each one has a separate form which must be filled out. None of them focuses on folk music. Do any of you use them?
We do buy ads in the CDSS newsletter & some folk festival programs.
Someone above mentioned having members like the opera does. We have members, but they don't come to our events as much as they used to. Possible reasons: cable TV & internet, longer working hours, aging & having less energy, "summer houses", i.e. going to a second home most weekends.   Many of our members now get together to make music with small groups of friends they met through our programs instead of coming to the larger group programs.
One other factor: 30 or 40 years ago we were primarily a singles group. As people got married, they did not go out as much, even if they married people they met at our events.
Another factor: The individual performers our members loved 20 or 30 years ago are not as likely to be performing now (many are dead, e.g. Utah Philips).
If we lose two people for each of these reasons, our event is not as well attended.