haiii..help me for the page expire

haiii...
first i have created a login page ..i had loged in with the existing
username & password..
after linking to several pages with that user name...i had signout
the account....
& returned to my login page...but here my problem araised ....
being in the login page ..i have clicked the back button in my
browser ... and just i am getting the previous user account with out
signin up... one more point..
when i click the back button in my browser the page is not retriving
from the server(postback is not done)

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