Cats and human love to have fun, no matter what age they are. The older cats and humans get, the less fun they tend to have. Make sure to have at least one "kitten/kid" moment every day for your emotional well-being.
Being polite is good, but being friendly is better, both with cats and humans. The easiest way to do this is I've found is to pretend it's an old friend that you haven't seen in a long time - this will make you feel and appear that you are genuinely happy to see them. Obviously don't go over the top screaming and hugging and whatnot, but using that feeling puts a real smile on your face and makes your eyes look happy. Between that and being honest, I never had a shortage of regular customers.
Kill them with kindness. When one of my cats acts naughty towards me, I respond by being nice to them. They immediately realize that they shouldn't be mean to me and apologize in their own way. This tactic works beautifully with humans, but you have to actually be nice - don't use a fake smile or nasty tone while doing it or it will completely backfire. If a human is being mean to me, I take a breath, then respond with kindness, such as: offering to help them with something, giving them food or drink without them asking, and even complimenting them on their appearance. I used to tell my staff, if you're nice to someone that's mean to you, then the worst thing that can happen is they complain you were nice to them and you won't get in trouble for that.
Speak slowly and clearly enunciate. They say that cats respond better to names that end in an "a" or "i" sound, so if you have multiple cats that have names ending with those sounds, to call a specific cat you would say their name more slowly and enunciate the syllables. With humans, there are so many languages around the world, but English is the official language of the business world - that way professionals only have to learn one language to make it easier for different language speakers to communicate with each other. The problem is that English is a slowly spoken language, whereas other languages like Spanish and French are spoken very quickly. When someone whose first language was not English and they learn English, they will use the speaking rhythm of their native language when speaking English. This unfortunately will lead to a communication breakdown with someone who is a native English speaker because English is not spoken that quickly. If an accent is combined with the increased speed, it makes it very difficult for the listener to understand what is being said. This will lead to miscommunication, frustration, and misunderstandings on both sides. When having a conversation with a non-native English speaker, both parties should speak slowly and enunciate clearly to ensure that both are being understood by the other.
If you ask a question, actually wait for and listen to the response. Whether I ask my cats or other humans if they need something, I always wait for the answer. For humans, this is a huge game changer in conversations. So many people have been programmed to have the typical small talk conversations that they will say the next thing in their "script" without even listening. When I ask how someone is, I refuse to speak again until they answer. Many times it results in an awkward silence until they realize that I'm actually waiting for a response, but the awesome part is when they do finally answer, I get to have a real conversation with them. They are generally pleased that someone took the time to listen to them, and are more likely to be friendly and helpful.
In situations where anger is the normal emotional reaction, I've found that disappointment is more of a motivator. If one of my cats does something really naughty, I squish down my anger response and replace it with disappointment. Why? Because the response to anger is more anger, it's easy! Disappointment, on the other hand, is more complicated, it's sad, it's depressing, and can only be resolved with apologies and good things. My cats have learned that they don't like to disappoint me, and will apologize profusely if they do and will usually be on their best behavior for a while. Humans absolutely hate disappointing other humans, and will even do so many wrong things so that they don't let them down. I worked in Loss Prevention for a while, and I was taught that I should be aggressive when apprehending shoplifters. I decided that instead of using anger, I told them I was disappointed with them, that their family would be disappointed with them when they found out what they did. You know what? Every single person I apprehended walked back with me voluntarily and apologized until the cops showed up. When people make mistakes, or lie, they expect anger because they can fight back with anger. When someone screws up and you give them disappointment, they don't know what to do, and most often they will do anything to make it right.