You may pay a compliment as you say “no”. Take a university professor, Delores, for example, when someone calls asking him to serve on their board, and he knows that he just can’t do it. He often say, “ I’m so glad you thought of me. I am a big fan of your orgnization, but my schedule just won’t allow me to accept your invitation.”
You may also decline in a positive way. For example, “That’s a good product, but it’s not something we can use at this time,” is a gentle way to say “no”.
Buy yourself some time. Seldom do you have to give a yes or no answer on the spot. Even if you feel strongly that your answer will be no. Buy yourself sometime by saying “Let me think about it”, “Let me talk it over with my family” and so on.