FormsAuthentication Logout not working

After logging out I can still go back to the previous page and view the secure page's content.  I need to stop this.  How do I do this?  Here's my code...
Login page:
                    FormsAuthentication.SetAuthCookie(Request.Form["Username"], false);
                    Response.Redirect("ClientMain.aspx");


Secure Page:
                    if (User.Identity.IsAuthenticated) {}
Logout Page:
                    FormsAuthentication.SignOut();
                    Sess ...