I make a point of thanking my clients and vendors year-round, but every so often, when I have an especially easy transaction, I will make a point of turning to the other person and saying "Thank you for making that easy, and thank you for being our [client/vendor/business partner]. I am so pleased to be able to work with you."
It's so easy to forget to say "please", and even easier to forget to say "thank you". In so many cases, our business etiquette has devolved to parting words such as "See you later" (or just "later", which I abhor even more) or "Have a nice day! Bye!"
While there is nothing wrong with wishing someone a nice day, I have learned over the years that simply thanking someone for their assistance with a transaction--regardless of which side of the desk I'm sitting on that day--can be a positive influence and make a huge difference in my future dealings with that person. I can think of worse things to have clients say about than "You're always so gracious." Would that all of my clients and vendors were overly gracious, because my business world would run much more smoothly.
I've learned to use courtesy to defuse my own frustration or tension during negative business transactions. There is nothing quite as effective at the start of a meeting to disarm a snarly customer or vendor as sticking out one's hand, looking them in the eye, smiling, and thanking them for taking the time to meet with you to discuss this important matter.
Most likely, they'll stammer something like "Uh, you're welcome...". While that won't necessarily make them stop being angry or frustrated or upset, it will generally help defuse the tension enough to open up a civilized conversation.
There are other ways to defuse verbal confrontations. I'll talk about those another month.