To Say No Kindly, Say Thank You and No
Skipping the excuse leaves nothing to argue with, and rehearsal makes it stick.

The Tim Ferriss Show · #885: How to Simplify Your Life — Tips from Oliver Burkeman, Chip Conley, Elizabeth Gilbert, and MoreElizabeth Gilbert, talking about how to turn down requests, describes a refusal she learned from her teacher, Byron Katie. It is thank you and no, with no but and no explanation.
That brevity is deliberate, and Gilbert says it is hardest to use with people she loves. Strangers who write out of nowhere get no reply at all, but friends and family get appreciation plus a clear stop, because she least wants to hurt them.
Start with thanks, then add no, and replace but with and. As Gilbert tells it, Katie calls the word but cruel, and says it is just an and.
Do not add a reason, even a kind one. Any explanation gives a persistent persuader something to solve, with a different date, format, or favor. A bare no offers nothing to grab and turn into a negotiation.
If the person pushes, acknowledge what you heard and repeat the no. As Gilbert tells it, Katie suggests replies such as “You might be right about that.”, “I see your desperation, and no.”, and “If I change my mind about this, I’ll let you know, and no.”
She described using it on a one-hour promotional video request from a professional contact. After Gilbert declined, the woman explained that sales were hard and she really needed the help, and Gilbert answered, “I hear you and I understand you, and no.” The exchange ended there.
With family, rehearse before you need it. Gilbert says she takes long walks and repeats “I’m not doing that right now” a thousand times.
Questions people ask
Isn't it rude not to give a reason?
Gilbert says that if the other person is a skilled persuader, any reason becomes something to get around. Say you are busy Tuesday and they offer Wednesday, or say you are tired and they say it will be easy. A simple no gives them nothing to bring back at you.
What if they call you selfish?
Gilbert uses Katie's line, “You might be right about that.”, followed by and no. She says they tend not to come back a third time.
How do I practice this?
Gilbert practices on long walks, repeating the sentence she expects to need. She does this with family requests, saying it a thousand times because she says it is scary to say.