As you exchange cards with people you want to stay in touch with, suggest a way to get together again and a topic about which to correspond. Send "nice to meet you" messages when you get back to the office. Build on your relationships via email and social media. Invite your contacts to share their thoughts and concerns with you. Help them in ways you want to be helped -- with useful links and introductions, for example. Give the other person as much as you expect to get.
Overall just be consistent. You don’t want to have a flurry of activity followed by dead period, and then try to restart the relationship when you need the other person.