Add these following 3 lines in Page_Load event of all the pages.
HttpContext.Current.Response.AddHeader("Cache-Control", "no-cache, no-store, max-age=0, must-revalidate");
HttpContext.Current.Response.AddHeader("Pragma", "no-cache");
HttpContext.Current.Response.AddHeader("Expires", "Fri, 01 Jan 1990 00:00:00 GMT");
What it does is, it asks the browser to not to cache the page. so when user hits the back button, browser will always get the new page from the server.