In a previous life^W job I used to be "internal quality auditor" (a two day/year gig) for an ISO 9001 certified company and one of the requirements for certification is to have a system to get to know your customers' opinion on the job that the company did for them.
So, we did interviews with our biggest customers, sent yearly questionnaires to the smaller ones, duly processed the results and used them to improve our way of working. (about three years after the slip ups)