It needs expertise and understanding of the customer and their needs.  To be a successful customer service professional, there are key golden rules that you need to bear in mind.

Have empathy. Look at the situation from the customer's point of view.  Placing yourself in the situation and circumstance of the customer can help you understand what the customer is really feeling at that point.  Only a thorough understanding of the customer's feeling can help you to resolve the situation amicably.

Believe that customer is the king. Don't be defensive or aggressive when customers complain about your service or product.  You have to be patient and listen to what the customer is trying to say.  Cross check at your own end to find out if there has been a lapse in your service.

Be honest and sincere. Do not unnecessarily cover up your mistakes.  Dealing with the issue upfront and apologizing for the mistake made will be better than arguing with the customer.

Improve on your soft skills. Be pleasant and polite when dealing with a customer.  Get a clear picture of what the customer is complaining about.  Be alert and take assistance if it helps in resolving the customer's query.

Be well informed about the product that you are working for. Without thorough and correct knowledge you will not be able to guide the customer or solve his/her queries.  It will also create a negative opinion about you among customers.

Use correct prefixes to address the customer. Be careful about the way you pronounce the name.  In case of written correspondence, write the names properly.  Wrong prefixes and spelling errors in name can offend customers.

Acknowledge the receipt of queries/complaints and indicate an approximate time by which the customer can expect a response.