Most of you can't say no because you are thinking that you are hurting someone. You are thinking that people may think that you're mean and inconsiderate. That is why you always say yes, that is why your position is always in a bad spot because of always saying yes. You can't take full ownership of your life by always saying yes.
The right way to say no is to say it fast, don't make any hesitation, just say it and say it without conscience. Say it like you really mean it and when you say it... don't ever make a reason for saying it, don't ever explain yourself. Just say it and when a person is still convincing you to say yes then that is the only time to make excuses.
The moment you say it and when the person convincing you stop... don't think about it anymore, don't think about yourself being mean, just move on to the next stage of your life. Forget what happened and place your focus on some other things. Because if you will keep thinking about what happened... that is the time you will feel guilty.
When you rejected someone, when you say no to someone... don't think of it as a sin, it is normal. You can make choices in life, you can make harsh decisions. And even if someone got hurt... don't think about it because you are hurting yourself even more if you keep saying yes but you don't mean it.
Once you say no, immediately place your thoughts to other things. Don't think about the person who got rejected. It is already past, it is time to move on. The one who got hurt is still alive and he can always heal as fast as he can if he will not take it personally.