A piece of advice I found helpful is to frame things in terms of expressing how people's actions make you feel. If you frame it as an objective or accusatory thing, people are more likely to argue or get defensive, but if you just say, "when you do X it makes me feel Y" (be specific) then it doesn't leave as much room to argue. The majority of people have an innate drive to avoid making other people feel bad, and a lot of problems can be caused by a lack of awareness rather than malice. But people also have a drive to avoid accusations of wrongdoing to preserve their ego and social status. So if you just bring an issue to their attention and say, "This is how your actions make me feel, do with this information as you will," it can often work better in terms of getting them to change their behavior as compared to being more forceful.

A lot of times when someone doesn't express when something's bothering them, they start ruminating about the person and coming up with all sorts of negative thoughts about them, even if the offender is well-meaning and simply unaware of what they're doing. This can lead to overcorrecting and blowing up in their face, or it can lead to pushing them away without ever giving them a chance. And avoiding it obviously doesn't get them to change their behavior. Counter-intuitively, being assertive can make people like and respect you more because people usually don't want their peers to just be pushovers. It's ok if they're defensive in the moment, not every interaction has to go positively at every moment.

If you follow the advice of framing it in terms of how you feel, it should make it easier to stand your ground. They can make excuses and that's fine, just stick to the core point. "I understand that [their excuse], I'm just letting you know that it makes me feel [emotion]." You know your emotions better than they do, so there's no reason for you to back down on that point. And once you've communicated your feelings, you can drop the subject and move on. Even if it looks like you "lost" the argument in the moment, it can still be effective at changing their behavior.

This advice works well in my experience for voluntary peer-level relationships. If they don't change even when they know it's bothering you, then maybe they're not worth trying to be friends with. But you shouldn't just assume that before you've brought it to their attention and given them a chance to correct their behavior.

How do you deal with involuntary relationships, like someone in a position of authority who you can't easily walk away from, who doesn't give a shit how you feel? Fuck if I know. Do a revolution and abolish hierarchy. In the meantime, you gotta ask someone else because I'm terrible at that. But I also used to be terrible at asserting myself in my personal relationships too and this advice did a lot to help me find the right balance.

This website is what I'm drawing from and I found it to be a great resource for recontextualizing how I thought about the issue. Assertiveness doesn't have to be about putting others down, and done properly it's an important part of a healthy relationship.