as someone that attended 5 of them this year as a employeer I can tell you this:
1. Don't come straight from class wearing jeans a t-shirt, pretend its an interview and dress like it
2. Just walk up and say Hi my name is <name> I am graduating in <month year> with a degree in <degree>. Do you have any current openings that I would have the skillls nessacary for ?
3. If Yes, offer your resume, hopefully the employer will have applications and job descriptions to give you.
4. Ask that they forward your resume to the appropriate person and ask if they have a business card so that you can follow up in a couple weeks and see where they are in the hiring process.
Most of the time the people representing the employer's at these fair arn't the ones doing the actual hiring, but pretend is if they are just in case. Even if they arn't directly related to your job (say HR people) they can at least give you a good review when they hand your resume to the manager if you stood out amoungst the crowd. The employer will take home a huge stack of resumes with them back to their company, make sure that whoever was at the job fair makes a good note on yours. (IE nice person, really interested in the job, willing to relocate, good interview candidate)