How to control "back" button.

I know this subject is really taboo, but I really need to find a fix to an issue.

I'm working on a site that has some pretty serious design flaws, but that is the potato I've been given and need to work within the design.  Most of the navigation is handled with session values, multiple user controls and AJAX.  Because of the design, the back button does not go back to the previous page, but rather to the very first page which is very wrong.

I need the ability to tell the browser what page was called before and control where the user goes.  How is this typically done? 

Thanks
Jay
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