Advertising is OK but now adays so much advertisements are ignored by the general populace. Adds on coin forums, like this one, would possibly be of some help but would only remind already coin show searchers or goers of that show. Around me there are 3 constant coin shows each month. Two are very successful. One is only average. Two are as yours are in size. One is usually 3 times that and every month and still growing.
Here are some of my suggestions:
1. Free parking
2. Free admittance for collectors
3. Coffee and snacks or even food available at a decent price
4. Have the shows on a Sunday from at least 9AM to 3PM. Saturdays if then become successful
5. As noted have people sign up for a mailing list. If they get a reminder post card, it actually helps since most of us have short memories.
6. Schools out now so a little late for this one. Have a dealer or yourself check with the school system if it would be OK to put on a presentation on coins for the schools.
7. If remote make sure there are plenty of signs saying coin show and where and when. And large enough to read while driving. This one ticks me off. I see signs for garage/yard/estate sales that look like they were made with a microscope.
8. Ask dealers if people with kids show up could they possibly have a cheap or junk pile for the kids to get free.
9. Check with coin magazine organizations if they would like to contribute some magazines for the show. Then put a table up front with those on them. Place a sign there to please only take one. The one show I go to has such a table and it gets emptied by 9:30 and it opens at 9.
10. Same table up front, place a bowl of candy, advertising flyers for the next show, anything else that might get someones attention. You could tell people that a flyer for their garage/yard/estate sale on that table is welcome.
11. Make sure the washrooms are kept clean or women will not come and if they won't come, they might find ways to stop their husbands too.
12. Request guest to not wear back packs since they bump into others in crowded aisles.
13. Post signs in grocery stores,
Walmart type stores, gas stations, etc. if allowed.
14. Once you have a start on these shows, make them consistant with a certain day such as the 2nd Sunday of each month. This again allows those of short memories to note the show is coming soon.
By short memories you may note that all politicians well know about the short memories of the general populace. That is why they don't bother with anything until right before an election.
15. Occationally have a coin auction also. Even if for a few coins at the start of end of the show.
16. A one or two coin door prise might also be an incentive. Even if only worth a few cents, there are those that really like that.