Disclaimer: I'm not an expert or anything, and I'm not always nice. But when I am nice, this is how.
- When friends get in an argument, don't take sides. Try to be neutral and unbiased. This way, neither friend will get mad at you.
- Disagree gently. If you disagree with something someone says, insetad of saying "you're wrong! This is the right answer," say "I think the answer is this, but that makes sense too." Or "I understand where you're coming from, but I think that..."
- Be polite.
- Just be nice, in the classic sense. Pick up people's pens for them, hold doors for them, help them with things, etc.
- Don't act superior to other people (even if you think you are). This makes you appear vain and mean.
- Avoid drama. Yeah I don't follow this one. You should avoid drama as in "but Becky said this!" but if your friend did something that upset you, let them know without making a big deal out of it.
- Don't make a mountain out of an anthill. There's no reason to blow issues out of proportion. If you do want to mention something to your friend, don't make it a bigger problem than it has to be.
- Don't be stubborn. Just because you're nice doesn't mean you have to be a pushover, that's true, but it definitely doesn't mean you have to be stubborn to protect yourself from people being mean to you. Being stubborn is NOT a route to becoming a nice person.
- Don't gossip. It's impossible, so I suggest you ignore this one.