First of all, the customer is not always right. However, the customer always thinks that they are right which can amount to the same thing. If they have their facts wrong then you may be able point that out, but opinion is more difficult. They may say that you have not delivered what you promised. Perhaps low quality, insufficient quantity or just not on time. This may be true, it may not.
Where the facts of the dispute cannot be established you may be left with the choice of whether to give the customer the benefit of the doubt or not. Doing so may retain their business but also leave you feeling aggrieved. On the other hand, insisting that the customer is wrong may result in them walking away. Ultimately, you need to decide how critical it is to your business to retain their custom. Part of that decision will be to consider what damage (if any) they could do to your reputation.
You may think that you are the only source of an exclusive product or service and that your customer cannot go elsewhere. But beware: if that is the case then it is almost certainly a luxury that the customer could choose to do without. Also, the more successful your 'unique' product or service is, the more likely that competitors will soon spring up looking for a slice of your success; and probably offering it cheaper too. By all means be confident in your product, but don't be complacent.
Sometimes a complaint will not be about the service or product but the person providing it. If you have employees responsible for customer relations then you need to ensure that they represent you appropriately. You may have standards and competence statements about what they do, but they also need clear guidance on how they do it. Does the customer feel respected by your representatives?
So what positive measures can you take?
Firstly, ensure understanding by being crystal clear on what you are promising, to what standard, and by when.
Secondly, build trust by listening to what the customer wants, demonstrating your expertise (without being condescending) and delivering what you have promised. Remember that for the customer, your marketing (which brought them to you in the first place) is a key part of your promise.
Thirdly, there's comes a time when you might need to refuse to deal with that customer or client; if they are costing you time and money instead of making it then you at least need to ask yourself that question.
Of course, by the nature of the topic, this article examines some negative scenarios. Think positive and remind yourself not to focus exclusively on the occasional difficult customer or client. Most are no trouble at all; and some are actually a joy to work with. Remember them too.