After you say no, let the silence sit. Filling it is how you end up saying yes.

Saying no creates an uncomfortable pause, and the discomfort lands on you rather than on the person who asked. Filling it means talking, and talking afterwards always softens it: an explanation, a maybe, a half-offer to help with something else. The other person did not change your mind. The silence did.

Next time, say the sentence and then stop. Count to five if you need something to do. The quiet will be awkward for about three seconds and then somebody else will speak, and it will not be you giving your answer away.