Scope creep on a fixed-price contract

On a fixed-price contract every unrecorded change comes out of margin. The changes that do the damage are rarely the big ones — those go through a variation. It is the small favours, each one reasonable on its own, that are never written down. The defences are a scope baseline the client has signed, a change log that records even the changes you absorb, and a habit of saying yes in writing. The third one sounds bureaucratic and is what makes the difference at the end. How do you keep that up without souring the relationship?