Rick Klau reminds us about:

a donut vendor in NYC who put his customers on the honor system, and in the process doubled his customers per minute (as compared to other, similar donut vendors).

What if you clients were on the honor system?  What would they pay you?  Next time you send out your bills, leave the total blank.  Let your customers determine the worth of your services.  What they pay may be more instructive than what they’d say.