Be patient and calm
Understand that the customer has a reason to feel mad at the company. Allow him to vent his frustration. Do not react to anything he says, keeping in mind that he is after-all a customer and therefore an asset for the company. Remain calm instead of losing your cool.
Talk very softly
Always talk softly or politely to the customer even if he is being rude in return. Customers will surely find their anger fading away when they hear the representative of the company listening to them patiently and replying softly.
Try to understand what the customer is going through. Try to feel his frustration and the extent of it. Do not take the disappointment or frustration of the customer to lightly. You need to make him feel that you do understand how bad his experience with the product has been and that you feel really sorry.
Do not argue with the customer even if you know that they are being illogical and telling a lie. Arguing will only tend to frustrate them more and make them angrier, which will consequently make it tougher to calm them and remove their grievance.
Respect your customer
Irrespective of what your customer says or how he conveys his frustration and disappointment, you need to respect him. Treating him badly will surely make you lose a lot of other customers.
Be logical and well-versed with your product
You need to have a comprehensive knowledge of the product that you customer is selling so that you can understand and analyse the problem faced by the customer better. This will allow you to logically deal with the customer and his problem.
Do not be too defensive
At times, you would know for certain that the customer is being illogical while complaining about the product. While it is wrong to point it out to him that he is being illogical, it is also wrong to become defensive and start taking the blame.
Give a freebie
Everyone loves free stuff. Giving a disgruntled customer a freebie is a sure-shot way of making him forget about his disappointment with the product and get him excited about the freebie that is coming his way.
Do not put the blame on a third party
You will make your customer angrier if you try to pass the bucket to the third party. The customer has come to you with his problems and he expects you to come up with a solution.