Have you taken advantage of your four walls and table tops to market this event? How about the wait staff? Price the event so you can offer five dollars for each reservation a member of your waitstaff brings in, guaranateed with a credit card. Market the event with table top print pieces and wall posters at your front door and in the bathrooms. Don’t forget to have an easy system for signup near the posters, and slips for the waitstaff that have spaces for all the information they need to collect.
If you have good day to day traffic in your restaurant, you should be able to fill up reservations by the wait staff selling them. You can also try radio spots. I don’t suggest radio for everyday advertising, but for special events they can work. Try a spot three weeks out, running in a good time slot for a couple days. Repeat the ad for several days start 4 to 5 days before the event.
Anyplace with a community calendar is a good spot to advertise also. Look to see if the newspaper is doing a specials New Years piece in the Entertainment section. Check for local magazine that the community looks to for entertainment information.