First off, let me start by making it clear that I don't pretend to be an expert in people management. I have however learned a few skills over the years that I find really useful. I hope they will be of use to you as well.
As a musician you will likely find yourself dealing with a wide range of people in your career: students, parents, contractors, event managers, fellow musicians, conductors, brides, just to name a few. Your interactions with these people and how you handle yourself can make or break relationships.
Notice I said "how you handle YOURSELF?"
That's right, good people management skills start with YOUR behavior, not theirs. You will never have total control over how someone else reacts, but you can absolutely have control over how YOU react.
Here are three rules I try to follow in my interactions with others.
Getting upset while someone else is upset does not solve any problems. Calmly listen to what the person has to say and then calmly state your position. If you feel yourself getting too upset, it ok to say "I need to step away from this for awhile to gather my thoughts and respond to you thoughtfully. Can we set up an appointment to discuss this further?" Chances are you will both have calmed down and the discussion can be productive.
Have confidence in your expectations and state them clearly. If you are not sure of yourself, people notice, and things definitely won't go your way. It's also perfectly appropriate to step away in this case as well. "I hear what you are saying and I need some time to think about it. Let's talk tomorrow after I've been able to put my thoughts together."
Even when you disagree with someone you can do it with kindness. Forcing someone around to your way of thinking is never the answer. Educating them in a kind and meaningful way will go much further and is much more likely to encourage a change in behavior in the future.