Collecting business cards at networking events and making notes about each person you meet is a great first step in building a network.
The next step is to start communicating with your new contacts to build the relationship.
After a business networking event take time to send each person you met an e-mail letting them know you're glad you met them at the event.
Refer to something they said or to a common interest you discussed with them. be sure to mention ways to work together near future.
Letting people know that you enjoyed talking with them helps them remember you -- and it helps you recall the reasons you wanted to stay in touch with them.