If you want to come home from the job fair with some real leads (not just free pens and stress balls), you have to do your homework first. Closely examine the list of companies that will be attending the job fair and identify your top prospects. Then go to the company Web sites to learn more about what they do, what clients they serve and the kind of positions they have available.
Pay particular attention to recent press releases posted on the Web site. These can usually be found in the "About Us," "News" or "Media" sections of the site. Companies typically issue press releases to brag about achievements, high-profile hires or future plans. Another good place to mine for current events is through company or employee blogs.
Obviously, you want to focus on positive news -- bringing up the latest corporate scandal won't ingratiate you with the recruiter -- but try to find something that reflects on larger industry trends. If you have time, search for more information through industry publications and Web sites. Come up with your own theory about how the current event you've chosen might effect the company's position in the marketplace.
When you get your chance to talk to the recruiter, mention what you've read about the company, ask what he or she thinks, then share your brief, insightful opinion.