Attire: Dress in a plaid/flannel (short or long sleeve) shirt. A suit says that you are a rookie and have never landed a job in your respective engineering field. No engineer in the field wears a suit to work.
Strategy: Speak with as many companies as you possiblely can. No reps there expect you to know anything about their company. Realize that each company there knows they are one of hundreds looking for help. Interns and co-ops are commodities to companies. You are an entry-level engineer without the commitment, as far as the companies are concerned.
Motivation: Imagine you are a company at a career fair to find interns/co-ops. You are judging everyone you talk to at a whim looking for someone that can bring something of any value to the company.
Approach: Be yourself and be curious. Companies display their products to spark interest and questions. Look at the product on display and talk about the first thing that comes to mind. For example the company that I was hired by: They had some circuit boards on display (I'm Mech Eng major) and I noticed that they had a conformal coating (waterproof) on the boards. One thing leads to another. If you are truly interested in engineering then be yourself and say what you are thinking.
What not to do: My company brought interns with to the carrier fair undercover. We were not allowed to say that were interns. We talked to and took resumes from all that came to visit us. Then we ranked the candidates on a scale of 1-5 with some comments. I was one of a few co/ops that had already spent 11 months with company and did well so they chose me for this task and trusted my evaluations.
Don't be intimidated. Be yourself. Pursue what interests you, not what you think companies are interested in. Good Luck!