I've learnt that in business a complaint from a customer is one of the most valuable messages that you can get. If the complaints dry up, your customers have given up on you and are buying elsewhere.

I've also learnt to be 'less British' and complain more. If done smartly, it can be very effective. A simple case was down at my local pub. The missus came to pick me up and the landlord said that we were ungentlemanly for not buying her a drink. She complained that the coffee was undrinkable, and two weeks later the landlord had significantly improved the quality.

This is a good example as the complaint chain was short, and the customer cared enough to bother.