Salesforce到.net应用程序的通知

We are building an asp.net web application which completely pushes the data to salesforce and is a forms authenticated website. to minimize no.of API calls to Salesforce and reduce the response time to end user in the website, when a user login, we store all the contact information in session object. But, the problem is, when some one changes information in Salesforce, how can i get to know in the asp.net web application to have the updated information queried again and update the session object. We are building an asp.net web application whic