1. Leave your expectations at home: This is meant to be pragmatic, not pessimistic. Do not expect that every job fair you attend will consist of on-the-spot interviews and preliminary job offers. Some companies are in between hiring periods, and are looking to gather a candidate pool to hire in the near future (see #3 for more details).
2. Company Recon: Plan which company tables to visit and set up a job profile on their websites before attending. Research what they do, how their current business year has been (even check stock prices, if applicable), and which positions they have open. Recruiters want to hear that you're genuinely interested in the company, not just looking for "any job" or internship. What will this get you? While it may or may not guarantee an interview, you'll look prepared, which does score points.
3. Say Something! Even if they don't have any current openings, talk to them anyway and get the recruiter's business card! Some companies attend to maintain brand/market presence - it is still worthwhile to drop by and present yourself. Recruiters do remember the candidates they speak to that fit their needs. I actually approached an international semiconductor table as an HR/Foreign Language major just to see if they could use any help in HR (even though the fair flyer stated they were only looking for engineers). As luck would have it, I spoke to their student recruiter who informed me that a few months later they might need some help. I emailed her my résumé, mentioned that we met at the job fair, and ended up completing domestic and international internships with the company over the course of the next two years! Imagine if I had just gone home since the flyer stated they only wanted engineers?!
4. A Recruiter is not a Résumé Drop-Box. Do not try to jump over the recruiter to get to the hiring manager standing next to them. If you impress the recruiter, they most likely will have you speak to the hiring manager, anyway! Additionally, do not just drop off your résumé and go; ask them a few questions about the positions you applied to, and ask them about their experience with the company (i.e. How long they've been there, how they would describe the corporate culture). When you're finished speaking to them, thank them, and ask for their business card (to send an e-Thank-You and attach a copy of your résumé).
5. Job Fairs are just one way of looking for jobs. If you felt you were unsuccessful at one, it does not mean you're unsuccessful in the job hunt altogether. Besides, they're a good way to practice controlling those interview jitters. This is just one method, and I encourage you to try other ones, too. For some suggestions, check out my post on Networking.