Do not say bad things about others behind closed doors: walls have ears and can talk.
Do not say you do not care when you slander others and you are outed; you become not only manipulating but, untrustworthy.
Never tell (particularly unsubstantiated) tales (except there are vital learning points, while maintaining anonymity), about subjects/persons you know nothing about; their relations might just be sitting right opposite or besides you. Rather ask questions to confirm the veracity or shut up.
Never come between two or more friends no matter how long you’ve known any of them.
Most importantly, never come between spouses, siblings, neighbors, etc., whoever you are, because those relationships are sacred. They will outlast you or you become the reason it didn’t work.
Finally, recognize boundaries so you don’t meddle.
Some level of self-control and discipline helps.