Building your network of people and keeping contact with them is very important whether you have your own business or you work for an organization. It is not selfish to keep contact with people so that some day they may help you get new business or employment. You are not exploiting them; in return you are constantly being useful to them in one way or another. In the same manner they are also using their network for business as well as personal gains.
The Internet makes it very easy to establish contact with new people and keep in touch with them. You can use social media and networking websites like FaceBook, Twitter, FriendFeed and Orkut to constantly remain in touch with them or you can write to them an e-mail occasionally. For instance you know that a person you met a few months ago is very interested in search engine optimization. If you come across a link that can help him learn more about search engine optimization you can e-mail him the link. Remind him who you are in case there is a chance he may not recall.
Calling may be a disturbance but if you know what would be a convenient time to call a person you may call him or her in case you have something useful to offer (something non-commercial of course) or in case you can ask something that he or she may know.
Read more about how to build your network of people and sustain it in this blog post at FreelanceSwitch.