On the short list of ways to get a job I am not convinced work for scientists, "Job Fairs" come right behind "Dropping your CV off at some internet black hole." This is certainly because I've been going to the wrong job fairs, and quite likely because I have no idea what I am doing there. Regardless, I am compelled to share with you my take and experience on this- I might not be great at sniffing out every job opportunity, but I have scored some interviews after job fairs.
I recently went to a "National Careers Fair" which is not as giant as you might expect. Regardless, I followed all the usual advice. Once the list of employers attending was listed, I look up the opportunities available at each website. I used that information to tailor my resume (in this case more transferable skills than literal science), printed an excessive number (in this case 5), put on my nicest clothes and headed in with a faceful of optimism. Of the 12 employers, I found 3 I wanted to talk to, and because I am a sissy, an extra one I wanted to "warm up with." (Tell me about the opportunities available at your insurance company...) It is really hard to just walk up to someone and explain who you are as a scientist. Even though recruiters at job fairs are usually very friendly, something about swimming in a sea of sweaty suited people tries my confidence.
The ideal interaction here would be for you to introduce yourself with a smile and a firm handshake, and then give the two sentence explanation of who you are that prompts the recruiter to want to pepper you with questions about what a great fit you are for the company. Lately I've been trying variations on "Hello, I am a recent graduate with a Ph.D. in molecular virology. I've spent the last 5 years doing drug discovery, and I am very interested in using my skills in drug development" or "I am a biologist with a Ph.D., I am very good at understanding complex technical information and communicating it and I am excited to help you better connect your clients with the information you create..." I find it difficult to be specific, but still imply I am flexible about the type of opportunities I am looking to pursue. For example, when I have tried the "Hello, I am a scientist, can you tell me about opportunities you have for people like me," the recruiter will usually panic- People like you? Who are you? Imagine being that recruiter, they are facing an unending sea of slightly desperate but highly skilled people and they are probably there to fill a variety of jobs. Don't forget your audience, help them help you.
My most recent success was when I approached a large Clinical Trials Contractor that focuses on drug development. Although the HR person was not recruiting people to perform trials (she was looking for healthy controls), she gave me the names of the places that usually award them contracts, since those large pharmaceutical companies might be hiring. Arguably, this is more successful than the interview I scored with a growing Midwestern software development company from the Pitt Career Fair.
Because if we are going to be perfectly honest about what happens at a job fair, most of the interactions I've had, the recruiter/HR person will recommend I check out the jobs posted on their website. As I said, I did that already. They might rattle off some variation from the About Us page, and they are vary rarely looking to fill a variety of positions that are appropriate for a Ph.D. They are really there to soak up people with Bachelor's degrees, or scout for interns. If you go to a job fair, which I still recommend you do to practice this type of professional interaction, don't expect to find a plethora of opportunities. This should liberate you to focus on what you actually want to get out of the experience.
However, I am told that major biotech hubs, like Boston or DC, have job fairs that are more targeted towards the Ph.D. set. I'd love more info on these, if you know of them or have been there.