To build and maintain a network of professional relationships, one should develop, maintain, and express genuine interest in other people and their work. Introducing oneself at conferences is a good start, as is recording and updating contact information, and setting up a system to remind yourself to reach out to people regularly. (There are various tools for this, ranging from a spreadsheet, to social network services, to CRM software. If you use gmail, you may want to try "contactually" for the mechanical part. )
Sending your contacts updates on your work is useful. And these should be targeted... it's ok to use a broadcast medium like twitter to announce new work to the world, but avoid sending out mass-email -- instead target those people who are likely to be most interested, and include a brief personal note that puts the new work in the context of their interests.
Most important, keep track of what others in your network are doing -- new publications, new projects, working papers, promotions, etc. (Social networks can be useful for this in some fields for tracking people directly, tracking working papers in your discipline, and disciplinary news can be quite useful, google scholar can also help alert you to new publications from people in your network. ) And when someone in your network is starting something of interest, re-connect -- with congratulations, a useful comment, and possibly some information on your own work (if it really is of interest with respect to their new project).