Practice saying "No" and its semantically softer companions: No thank you. I'd rather not. I don't think so. Not just now. Maybe another time. I don't care for that. Thanks for asking, but I must decline. Or, flex your muscles with its semantically less friendly amigos: When hell freezes over. Maybe when pigs fly. Take a flying leap. In your dreams bucko. Not in this lifetime.
Either approach is preferable to not saying "No" when it is your true internal response. It is much worse to avoid, deny or manipulate the conversation and the circumstance than to speak the truth in whatever manner you feel most comfortable speaking it.