Customer service is important to businesses as it almost guarantees the fidelity of the customer and their return to the service or store. Many companies perceive customer complaints as a measure as to what needs to stay the same or change in order to maintain customer satisfaction.
Therefore good and diligent customers take customer service complaints seriously. Nevertheless, the kind of complaining can also determine the kind of results one achieves and how you are perceived.

As in all relationships and interactions with people more generally, it is essential to remain polite and calm despite the grievance one has. Treat and talk to others the way in which you would like to be treated. Managers will be more than willing to address your concerns. On the other hand if you become too loud-mouthed, aggressive and impolite or lose composure, the other people will most likely feel verbally attacked, threatened and disrespected. All this will turn against your favor and you will get a defensive response from the manager.

People generally respond better if you are not directly attacking them or blaming them for their or the problem. A reassuring phrase to employ would be saying something to the effect of “I’m sure the person in question does not represent your policies”.

Similarly, you could try resisting the temptation to complain first but start off with a complement. State that you are a loyal customer who has generally had a good experience with the services and are nevertheless disappointed at your current experience. You could express how you would like to return and continue being a customer however you would not do so if he cannot honor your request.

On top of expressing your concerns you could propose a solution. Be a negotiator of resolutions and you could generate a positive, reconciliatory result.

Demanding free services, meal or drink will make you seem unreasonable. Stay courteous and reasonable as well as realistic.