There are several things I'm grateful to my mother for and having good manners is one. She always told me when I wasn't behaving properly and followed with a good example. I'm not saying this is how everyone should behave, I'm just saying that this is how I behave.
1. Always arrive on time or early
Whenever my mom had to be somewhere for an appointment, we were always early. We'd arrive thirty to forty minutes early every time. To this day, I'm always early to appointments or social events and if I'm late or someone else makes me late, it really annoys me because of this ingrained moral I grew up with.
The reason behind this is so that you won't miss anything at your destination or make anyone else have to wait on you to arrive to begin.
2. Always Be Kind To Your Waiter and Tip!
There is very little else that I dislike more than someone who is a complete douche to their waiter. This person is being paid to serve you, yes. But, if you feel like you can treat them with complete disrespect, I will never eat or speak with you again. It doesn't matter how nice you are to me, if you're horrible to anyone in the service industry, I won't associate with you again.
I remember growing up and we dined with a family after church on Sundays frequently and every single time one man would start a huge scene saying how horrible the food was or some other complaint until finally he'd get the whole meal and that of his family's for free. It didn't matter that he ate everything and loved every bite in reality, he just wanted to make enough noise so that he'd get it for free. This was the perfect example growing up that taught me never to do this.
Tipping is also important because you never know if all the tips are being split amongst everyone. If you have really really horrible service make a formal complaint to the manager but still tip (and tip well)!
3. Never say, "No offense, but..."
9 times out of 10, if you say "No offense, but..." "This is only my opinion, but..." "I don't mean to insult, but..." or something equivalent, anything you have planned on adding at the end of that but, will be offensive. Like I said, this isn't always the case, however if you find yourself about to say something that will piss someone off, keep it to yourself.
My general rule of thumb is if I have to preface something like that, I'll just keep it to myself. And probably tell my husband later because he loves me and knows I'm not a terrible person. This applies to both in real world situations and online.
Though, I have replied with those words before online in response to someone who used it on me before in a snarky sarcastic manner to maybe open their eyes on how that looks... Sadly, it didn't work and the original comment poster became offended! Imagine that!
So, just don't say it or use it. If you have something you need to say to someone that you know will make them angry and you have to say it anyway (who knows why), just say it instead of beginning the statement with something stupid. And if you do want to preface it with something like that, then don't get mad if that person gets offended. A bit unrealistic, no?
4. Know When To Speak Up
If you witness that someone is being mistreated, know when to speak up in order to make an impact.
This is very situational. This can come in handy when you witness sexism, racism, or just a regular inappropriate correspondence.
For example, if you see a woman being mistreated in a sexist manner by a huge group of men and you're a woman as well, go to the HR department and make a complaint if it's just comments. If there is violence, call the police. But, speak up.
If you witness any form of racism, speak up. Don't let that crap fly. If violence is involved, call the police.
But lets say, someone you've never met has contacted you about a personal matter with your family. You've never met this person. You've never spoken with this person. And yet, they have a need to let you know exactly what they think.
For example, my estranged father's wife (they got married shortly after my parents were divorced), sent me an email telling me it was time to forgive my father and I was being childish in refusing to be a part of his life. I responded in a very polite manner telling her it wasn't her place to tell me what I should do, but that it wasn't a matter of forgiveness (which I already have) but that I had other reasons for not being in contact with him (namely not wanting to be a part of a toxic manipulative relationship).
That is when you should know not to put your two cents in. If you've never met with the person before, or even spoken with them before, don't butt in because your opinion doesn't matter to them. And sadly, you might actually make matters worse.