How to save the input values in an unfinished node form Drupal 6:

Is there a way to save the values in a node form, so that if the user leaves the page before the form is completed, they can access that form at a later time and have the values that they input still in their respective form fields?

Can you use node_api to somehow save the node as "unpublished" once the user gets to a certain point in the form? How would it work if required fields aren't yet filled in at the time of the pre-save?
You could save the values in the users session ($_SESSION) and clear the value(s) when the form is submitted. You would have to use AJAX to update the values whenever a field is changed, as you can't do it when the user decides the leave the page.