Don’t collect the business cards of absolutely everyone you meet – there is no way you’ll be able to follow up on that many people. You’ll get a feel for which of the people you speak to are genuine, and therefore ‘qualified’ leads, just as easily as you’ll work out who the tyre-kickers are (you know the type, those that hang around your stand and really just want to see what freebies you’re giving away).
Only collect the details of those who are interested in what you do and look like they could be potential customers. Don’t waste your time on those who didn’t show a true interest.
As with all marketing, target your efforts towards those with the greatest likelihood of becoming a customer/client.