After you have said it

Do not add to it. The commonest mistake is the follow-up message an hour later that softens the position or reopens it, which teaches the other person that your first answer is a draft. Say it once, warmly, and then let it be uncomfortable for a moment. The discomfort almost always passes faster than expected.

And expect to be wrong sometimes. Some noes are mistakes and you will find out later. That is a normal cost of having a position at all, and it is much smaller than the cost of having none — which is a calendar filled entirely by other people’s priorities and no way to explain how it got that way.