Keep a pen in your pocket. When you meet a potentially helpful contact, ask for their card and write info you want to remember on the back. Spend a few minutes with each person and move on. Listen more than you talk. Try to follow up with a letter or email a few days later.
Most people have a hard time talking seriously about business in a gathering.
Make your contact, then get back later to answer questions and make sales. Answer E-Mail Fast and Effectively Email is quickly changing the way we do business. Many customers now prefer it because of email's speed and convenience. The downside is you can get swamped with too many messages. You could ignore email, but business will suffer.