Autosave form fields example
Try the Save Form State module. Here is a quote about it (from the module's project page):

... provides a way to autosave data entered in any Drupal form without actually submitting the form, which helps if the user is writing an article or a comment and the browser crashed or the power went down, or even if the window was closed accidently.
It works using the jQuery Sisyphus plugin, which is a lightweight jQuery plugin that uses Local Storage to save form fields every specific time span that is configurable from the module settings page.