One method can be, on every survey completion trigger an email to respondent using workflow email task. The email should have the retake link of their current response. We can even mention in the end of survey message to check email to get the survey retake link. 

@alexmessinger I agree with Shashi, as long as participant has submitted the full survey you need to issue them a retake survey link.  Otherwise if you instruct participants not to submit and you have enabled option to save and return, then participant could come back and change answers.