if it is an emergency and for a client you haven't dealt with before - contract, terms of sale, before the work starts
problem is that best of intentions are often forgotten - when the fire is out.
i agree with the video - get paid then and there. get authorization, credit card, whatever
you don't want to be in court chasing unpaid debt. only the lawyer wins.
set out expectation before you start work - ie when the pain is still current. and get it in writing.
but this must all be done BEFORE you start - anything.