This might seem to some like I'm stating the obvious but, don't wear jeans or pajamas! This isn't a casual affair for most employers this is them casting a net to potential employees and if you want to be thought of as such you should dress the part. The same goes for wearing clothes that is too big or too small, you will look sloppy and unkempt. If you think you look that way people probably see what you see especially when they are trained to do so. You can wear a suit but, if you don't have one or want to save that look for a potential interview you can try out some of the styles I've illustrated below.
These are just a few general ideas of what you could wear. I wouldn't say that you have to dress in completely formal business attire to attend a job fair (although, it wouldn't hurt.) Perhaps you don't even own a suit yet (ahh, college) either way, you do want to convey a sense of professionalism in what you are wearing. The person you meet at the fair, whether hiring manager or recruiter, is your ticket in and you don't want to make the wrong impression!