Spend time with Russians!!!

Or the Japanese, for an opposite perspective.
At least that’s how I did it / how I learn best… from being in environments where such a thing is both normal & expected.
- Russians taught me that both yes & no are entirely neutral. Neither is positive or negative, in & of themselves. (As opposed to being in the US where saying “no” is often taken so far as to be fighting words / a character attack / or an insult at best. And if one
does desire to say no? There is a complicated verbal dance, and submission/dominance display involved, except for in certain subcultures.)
- Spending half my childhood in Japan? Taught me 10 zillion ways to say “no” whilst NEVER actually saying “no”. (Yes, there is a word for it in Japanese. Only gaijin really use it. Personally, I only ever use it as a command for my dogs. There IS, however, a tremendous spectrum in how politely or rudely one wishes to express the idea of no. One of my favorites happens to be “I will consider it, for an eternity.” Which is an enormously dismissive f*ck you.).
Failing cultural immersion to normalize where you want to be?
Cheat. Sheet.
Come up with a dozen different ways to say no, without actually saying no. The more honest the better. From “I wish I could, but” to “I’m not sure that’s possible at this time, with my ABC, but it sounds exciting/clever/smart/useful/important/whatever-adj-best-fits” to “Oh! I’d love to, but I couldn’t undertake it myself, let me get you in touch with…”