Form data is not "remembered"

Hi, I have a page with a couple of form fields that are being POSTed to a processing PHP script. The page that contains these form fields is itself a PHP page that uses sessions and so on. I have to make "server-side data validation" on the fields because of their complexity. When the user hits submit and comes to the result page, if there was any error I ask the user to go back to the form and correct the mistakes, using either his browser BACK button or by clicking on a link that does a javascript:history.back(). The problem is, when the user gets back to the form page, all of the values he had typed in are gone, and he has to start all over again. Does anyone have an idea why this is so, and how to circumvent it without saving the user data into a session variable and then setting it back again once the user hits the form page again ? I have seen many pages where your previous data would stay in the form after you make a "Back" operation, but here, it disappears. Thanks for any ideas, and have a very nice day.