Should not logout after clicking the back/forward button

Hi all I have wrote a code for login now. User is able to login but after logging in when he clicks on the back button user automatically gets logged out. very next if clicked the forward button user gets logged in..!! how can I solve this issue? What can I do. Please help. Many thanks..!!! Sorry for the inconvenience that am unable to paste the code here..!! Hope you understand..!! thanks...!!