If someone else is monopolizing the employer's time, try to make eye contact with the rep to let him or her know that you're interested in speaking. If all else fails, move to the next exhibit and plan to come back later. If you discover a real interest in an employer, find out the procedures required to secure an interview. Show enthusiasm, but don't go for the hard-sell approach. At some career fairs, initial screening interviews may be done on the spot.
Sincerity always wins.
Don't just drop your resume on employers' display tables. Try to get it into a person's hands and at least say a few words. I'm very interested in talking to you. A little advanced preparation goes a long way and can make you stand out among the masses of other attendees.