The shape that works

Three parts, in order, and short. Say no clearly enough that it cannot be misread. Give one reason, not four — several reasons read as excuses and give the other person a menu to work through. Then, if it is true, offer something you can actually do: a smaller version, a later date, or the name of somebody better placed.

The most common failure is burying the no. "I would love to, it is just that this week is difficult, although possibly later, I am not sure" is heard as a maybe, because it is one. The kindest version is the unambiguous one, delivered warmly. Ambiguity is not politeness; it just moves the discomfort onto the other person.