Seth Godin wrote a very excellent blog post on How to Deal with an Angry Customer.
I became a fan of Godin after reading his book on marketing, Purple Cow, which I HIGHLY recommend. It's not a dull business book, it's a very quick read and full of great ideas on how to make your business stand out.
And speaking of angry customers, Godin addresses this, but I'm going to say it again - Don't get angry back!! This is the most important thing and a mistake that so many people make. The nice thing about SL is that you don't have to maintain any happy facial expressions while dealing with the angry person. You don't even have need to regulate your voice. You just need to make sure you type nice.
Another thing that I've heard recently is "The customer is always right." I hate this phrase. It should be "The customer is often really wrong and misinformed, but I should still treat them with respect." I guess it's not as catchy. But really, as I think we all know, sometimes customers can be dead wrong about how your product works or what they were supposed to get. The key is to realize that you can't bend over backwards to make every customer happy. Sometimes you will not make that customer happy, but that doesn't mean you shouldn't try really hard to please the customer. At least make them feel like you made an effort.