Is it possible to change your policy and possibly send out a letter to all clients balancing out your gratitude with the policy change?  "I am so grateful to run a business that is valued and has such quality clients.  To keep my business flowing smoothly, I have instituted a new policy of requiring payment at the time services are rendered.  Thank you for your continued support.  I look forward to our continued relationship"  This would also apply to new client.  A clear understanding Before the work begins.  I find that when I'm clear at the beginning, this helps to get what is needed and then move on.  Backtracking wastes a tremendous amount of valuable time.  This applies to staff as well.  Being clear avoids confusion or guessing and doing things that aren't necessarily helpful.