One of our favorite restaurants that we would go to ALL the time started suffering in the service area. We noticed over the course of about a year that it was definitely sliding down hill. Since we always paid via our AMEX account, it was easy to look up and see exactly how much we spent in the last 12 month. My spouse wrote a letter, outlining the bad service, how much we spent, and that we wouldn’t be dining there nearly as much as before.

The restaurant sent us a apologetic letter back with a gift certificate to the restaurant. The next time we went we took the certificate and were giving it another shot.
Wouldn’t you know it, the service was bad. Table next to us was seating about 20 mins after us and had their food and where half way done before we noticed it. (Now you should note it was Halloween, a busy night for the place; so we were cutting them a lot of slack). The waiter and then the manager came over and apologized, seems our order got lost in the shuffle. At that point I explained to the manager that I thought it ironic as were where there that evening to use a gift certificate that the restaurant had sent to us because of bad service.

She immediately knew who we were as our letter had been used in their last all staff meeting.

The service did improved after that and we still visit the restaurant, just not as often as before.