I'm not talking about the yes, sir and no, ma'am practiced at your custody hearings. I mean actual courtesy.
Please and thank you do top the list. How hard is it to add please, and say thank you? Olympic gold medal hard, if a visit to Walmart is any indicator. Start at home, and practice with your pets. Say please when you call them to dinner and thank you when they come inside after doing their business. The animals won't mind, and it will become habit for you. Using those words regularly and without sarcasm softens both your voice and manner. While you're at it, try to mean the words. People know genuine courtesy when they hear it.
Think of others. You're not the only person who uses paper, so make the time to refill it, or any other supply you notice may be lacking. No, you don't always have the time right then, but do it when you do. God will notice, even if no one else does. Hold the door for the person behind you, man or woman. Nothing in your silly life can't wait 2 minutes. Help the person at the grocery store reach the top shelf. That's why God made you tall.
Make requests, not demands. Rarely do you have the authority to order another human to do something, unless you're a drill sergeant. You can ask them to do something, and if you ask nicely, and it isn't a ridiculous request, they're likely to do it. You're the same way; you know you are. We all like to be asked, not ordered.
Open your mouth when you speak. Your words are important. Don't mutter them. Don't garble them. Enunciate. Mumbling is dragging your mouth the way shuffling is dragging your feet. Both are lazy and discourteous. Don't do it if you are physically capable of doing otherwise. Most of you lemmings are, if you'll put your phones down long enough to let a thought form.
Do unto others as you would have them do to you. That actually is in the Bible: Matt 7:12, unlike many of those pithy proverbs Grandma chirped at us like "God helps those who help themselves" (patently false) and "Cleanliness is next to Godliness" (you made that up). The Golden Rule is about courtesy. The world isn't here to serve you. Quite the reverse, Jesus would say, so be polite in your service.
Courtesy dovetails nicely with calm. Practicing genuine courtesy while remaining calm is a combo that will fast-track you to management.
Caveat: courtesy alone isn't worth much. The incompetent can please and thank me to death and fail to solve my problem, which leaves them in the dust as I seek satisfaction elsewhere. Competence has a higher XP, but without courtesy, competence leaves customers cold.
Applaud the jellyfish.