Trouble getting my back/forward buttons to work correctly

My program has 4 pages that have session variables on each page and they are retrieved on the 4th and final page then submitted to a database when the submit button is clicked. When i was testing my program i filled out all 4 pages and then clicked back to refill-in something on a previous page and all my information wasn't filled out anymore. So i added the code in the page load sub btnBack.Attributes.Add("onClick", "javascript:history.back(); return false;") and this cured that problem so i could click back and the data would still be filled out. Then i saw a new problem....after i went back to fix something on previous pages 1 and 2, i would click the next button to go forward to pages 3 and 4 that i've already started filling out and now the data on those pages were cleared. So how do i make it so that the information from the whole program stays filled out whether i go back and forth from the 1st page to the 4th until i c ...