I did a quick search but did not find any results, I apologize if it has been suggested before.
My suggestion would be to allow an end user to Save a Form to edit later. To avoid getting in to user management, login, members area... Maybe just ask the user if they want to save, to provide their email (if it was not part of the form) and email them a unique url to the form that populates the fields based on what they previously submitted. The user could just click the link from their email and continue with the form.