Use master page to your website/web application and make all the pages to its content page. design your master page, and place logout button over master page, so it will be visible to all your pages.
please refer this links for more information.
http://webproject.scottgu.com/CSharp/MasterPages/MasterPages.aspx[
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You can add master page there is not matter weather it is website or web application, if you have completed your application/site already without using master page, even then you can add master page to your application/site, just few design changes required to your existing pages.