Back button using sessions

Can someone tell me how to get around the following problem: I am using sessions to store a person's ID and password that they authenticate with at the start page. The entire web site is secured this way. The problem is that hitting the back key often gives the "This page has expired, hit Refresh to Reload" kind of message. How can this be avoided? I've written the same application without sessions (unsecure obviously) and I don't get asked for refreshes at all. Thanks, Troy Moreland