Most clients /customers want to feel like you're listening.  So actually listen (don't just wait for them to stop talking so you can tell them your opinion). Do your best to be patient and don't lose your temper; it resolves nothing except driving a client/customer away.  And know how  much your competitors' are charging. If you are charging a lot more than everyone else is, for the same service, then offer a discount with better service.  You don't have to be cheaper than everyone else, but you must be competitive.  And, in a time when so much is done impersonally and online, human interaction & a more personal customer service can make the grumpiest customer  feel better.  And finally, if you have a client that is truly a nightmare to do business with, then complete the job and add him/her to your personal 'blacklist'...as someone who you refuse to ever work with again.