Be ready for their rapid-fire questions with your rapid-fire responses.
Unless it is a small or limited job fair, you will want to plan to spend the entire day there. You should always spend time in advance researching the employers that will be attending, not only to decide which you have an interest in, but also so that you are fully prepared for those with which you meet. The very best time to attend is early in the morning and then again late in the day. Even at some of the commercial job fairs (which are notorious for long, long lines), by arriving early you can usually beat the lines and meet with the most popular employers first. During the "prime time mid-day crunch," you can usually expect long lines and lack of quality time with the recruiters. To estimate how long your wait will be, simply sample the average amount of time the recruiter is taking with each person, extrapolate over the number in line and you have your answer. At a recent job fair, the recruiter was taking five minutes with each candidate and there were twenty people in line. How's your math? That's over an hour and a half waiting in line for a five minute meeting.
A good strategy to follow is to meet with the most popular employers early in the day, before the lines develop, and then talk with the "second-tier" employers during the main part of the day. Then before you leave, make one more contact with the employers in which you have an ongoing interest. With proper planning and strategic timing, you can usually avoid the long lines and make your time more productive.
Think of it as a day at Disney World. If you can't see it all, make sure you at least see and experience all the highlights.