I've had no end of success by simply slapping a name sticker over my coat, wearing my side-backpack/laptop-bag, both of which because I usually have limited time before class, and simply having fun.

Booth-sitters are usually bored, know less about the details of the job you might get than the people you'd be working with (they're often HR), and have dealt with many kids wearing suits and sounding important. But they are there to be the first line of defense. So be yourself, someone they could work with and maybe even like, rather than someone who sounds like everyone is trying to sound like!

And bring a resume. One page. If you can't sell yourself in half that, they won't read the rest, so make your point and make it fast.