A disagreement format for holding a firm position without escalating the relationship: (1) state your position clearly, (2) don't require the other side to agree with your reasoning for holding it — only that you hold it, and (3) deliver it in a friendly manner. It separates the substance of the disagreement from the tone of delivery, which is what makes it repeatable under pressure.
The delivery half of the technique depends on Congruence Principle (Words, Tone, Body, Eye Contact) and Body Language Reveals What Words Conceal — a "no" only reads as friendly if tone, body language, and words actually align.