how to say no and mean it.

in a world where yes means no and nos mean yes. how do u let another person know that your no actualy does mean no. and that you seriously dont want to do what they are asking u to. how do you let them down without a. making them feel bad. b. feeling guilty c. giving a shit. why does it happen that when someone does a few things for you and the time they dont do one thing why do you get mad/upset at them. and why does it happen that when someone never does anything for you not even smile at you, the time they do one thing for you, you are grateful. is it a case of expectations versus no expectations? Can having no expectations actually be a good thing?