Figure out Staffing
You're going to need a lot of people to help make your job fair a success. In advance, you'll need a marketing person to ensure job seekers know about the event, and to find sponsors; a logistics coordinator for the venue; a graphics designer and an employer coordinator to reach out to companies to participate.
On the day of the event, you'll need ushers to greet attendees, reception staff to register them, perhaps parking attendants, staff to build up and tear down the booths, and maintenance workers to clean up afterwards. Determine which positions can be filled by volunteers and which ones you might need to pay for. The U.S. Agency for International Development (USAID) offers a helpful brochure on the responsibilities of these roles.
During a slow economy, there will be lots of job seekers but not many job openings so companies may not wish to participate in a job fair. Your employer coordinator can remind them that this is a good way to keep their name out there and network with future employees.