Forms lose data when going "back" in browser

I have a submission form distributed over 5 pages. When the user hits the "back" button in the browser on one of the pages to make changes on a previous page, all fields are emptied. This is only when I have a session running. When I use the same pages without the session, the data stays in the form. So I understand that the session expires the page immediately after leaving it. Is there a way to give the page a few minutes before it expires or a similar behaviour? Thanks a lot. -Zavier