Firstly, you should have jotted some notes on the cards you received so you can follow up intelligently. It doesn't make a good impression if you get their spouse's name wrong in your follow-up. So, secretly, write notes on the backs of their cards so you can accurately remember details.
Prior to the event, plan your follow-up. Diarize your follow-up schedule so you'll have no excuse to procrastinate in taking action. Your follow-up period should be no more than a week for your first contact. Start off with a simple email. Thank them for their time and try to find something interesting to share with them. It could be a white paper, or an article which is relevant to them.
Depending on how much potential you see in the contact or the value of something the contact gave you, you may want to send them a small gift. Send Out Cards is good for this (ask me if you don't know anyone) or you can do it yourself. Perhaps a small value gift card, or, if something they gave you (perhaps a really hot lead) was exceptional, think about spending a little more.
Other things you can do is share event pictures with them if you have them. Write introductory emails if you passed on a name to them. If you know of other networking events that are coming up which may interest them, share the details. Your goal is to stay front of mind as a valuable source of help and information without them thinking, "Oh no, not ANOTHER email from Ian!"
Collecting cards does not make the phone ring. Following up those cards does!
