We'll cut to the chase. It’s boundaries, or more specifically, setting boundaries liberally and respecting them consistently.
Surprised? Think about it this way. Boundaries come on a spectrum, which looks different for each person-- there are smaller boundaries, which might look like, “Please lower your voice; my roommates are sleeping” and bigger boundaries, which might be more like, “Please don’t come to my house again.”
What a lot of folks don’t realize is that setting and respecting smaller boundaries are the single best way to avoid the big boundaries.