The best part about customer service is to listen to the customer. If he/she complains, take it positively, because most customers will not even bother to complain.
“Thank your customer for complaining and mean it. Most will never bother to complain. They’ll just walk away.” – Marilyn Suttle
Segment your customer and understand how they want to be heard. If it is at the point of purchase or a service centre, the best method is to use a simple customer feedback device like Mpulse. User can just click a smiley to register his/her feedback.