Job fairs don't allow much time for interaction with the recruiters. With a long line of impatient applicants waiting behind you, you need to make an impression and make it fast. Here's where a little research will go a long way.
Before you arrive at the fair, figure out exactly what kind of applicant the company is looking for to fill its open positions. Pay particular attention to the list of desired skills and job experience included in the job listing. Do your best to match those skills with your own. When it's time to introduce yourself to the recruiter, highlight those skills and experiences prominently, then ask, "How would you envision someone like me fitting into your company?"
By asking this question, you are encouraging the recruiter to think of you as already being on the job. Instead of telling him or her exactly how you'd contribute, the recruiter gets to put the pieces together for him- or herself. If you've done your homework, then it will be clear how someone exactly like you would be a great addition to the team.