Best way to track who is accessing a record

I have an application that has a SQL back end, and I want to be able to track who is accessing a record so that no one else can access it at the same time. I was going to do this with a table or application state, but how can I avoid keeping files locked when someone abandons a session? Any Ideas?There are third party tools to do it but try the links below for code to track updates and links to the third party tools. Hope this helps.
http://www.aspfaq.com/show.asp?id=2448