How to persist user selections in webform

Hi,
I have 2 web forms. The first one contains dropdownlists. The user can make selection and click a link to launch the second web form. Once the second web form is launched, the user can click the browser's "Back" button to go back to the first web form where all of the selections are reset to the default.
I try to save the user selections in ViewState so when back to the first web form, the user can see the previous selections.
The question is when to save to ViewState and when to retrieve from ViewState.
Thanks.