Spencer,
My numble opinion;
1. As above weather is important unless you have a warehouse.
2. Plenty of notice, secure the dates early check whats on in the area don't clash with another show or if near a sports venue stay away.
3. Toilets sound simple but some local events have none, share or not enough.
4. Food reasonably priced - i approach a local truck stop burger a few years back in my area and they were glad for the business.
5. Advertise nataionally, local and surrounding counties, including caravan/camping sites, cadet forces, schools, local St Johns ambulance (first aid cover tick in the box)etc.
6. Goodies - car stickers or pens with car show name and year, free entry into prize draw (local firm sponsorship) or award best club/best car etc.
7. Cars - sounds daft but get some confirmed/garanteed attendees.
8. Well sign posted on the day.
9. Marshalls/guides - to help and provide assistance.
10. Joint venture - have it the same day as fairground, market, car booty etc in the next field one location two events more interest to a wider audience.
Quick attempt there are many more I'm sure. Let me know were and I'll try to get some work in the area.
Best of luck regards
Mark