I received an email from a student this morning, as follows: "I met someone from (name of organization) on campus at last month's career fair, and she mentioned they might need help." The email concluded with the request that I connect the student with the organization's representative, seeing that he didn't know her name (much less spell the name of the organization correctly).
This request was not unusual; in fact, I receive one or more like it almost every week. The solution is simple: when you meet a person at a business event, get their business card. Perhaps even write a note or two on the reverse side of the card to capture the key points of your conversation while they're still fresh in your mind. The bottom line here is to have a physical record of contacts you make so you can follow up as appropriate in conjunction with your broader job search/career development efforts.
More importantly, the business card offers a concise, permanent record of having met (or in some way encountered) that person that you can refer back to later. Even if you choose to scan the card for inclusion in an electronic list, the hard copy serves as a point of reference and even reflection on an encounter that in time may prove to be significant in one's career.
In short, always get someone's business card when you meet them and store it in a secure place. You never know when it might come in handy.