The best time to set a boundary is before you’re angry about it.

Lines drawn in anger come out as accusations, and the other person hears the anger rather than the line. Drawn beforehand, calmly, the same sentence is simply information: here is what works for me. The reason it so rarely happens that way is that nobody thinks to set a limit while everything is still fine.

Think of one thing that is heading toward annoying you: rides you keep giving, a group chat at midnight, a friend borrowing money. Say the plain version now, while you still feel warm toward them. It will sound like a preference instead of a complaint.