Beware with session variables, I've found that if a variable is set on page 1, then the user clicks through to page 2 and the variable is set to another value, you have to be very careful of the back button. In the scenario above, if the user clicks the back button to page 1, the session variables are reset to how they were on page 1.
I'm not sure why the should be any issues with the back button. I'm assuming that you have the following scenario: Page1 has a form that gets submitted and then page2 checks to see if the previous form was submitted and then saves the form variables to session variables.
If the back button is pressed and the user goes back to Page1, it will have no effect. If they submit Page1 again the session variables would be changed to whatever was currently in the form fields, which would obviously be what you would want it to do. If somebody went to Page3 and then clicked the back button and went to Page2, the session variables wouldn't be changed because the logic on Page2 checks to see if there was a form submit from Page1.
I use this kind of logic all the time, so I don't see why there should be any back button issues at all. It would be really ugly and unwieldy to try and maintain a flat file to track user variables. The key is: don't set the session variables unless there was a form submit from the page you want to grab the variables from.