Decline politely

One of the things I need to learn is to decline politely things that I don't want to do. Sometimes I feel compromised to do something only because someone will be happy if I do the favor.

The problem is that I don't want to hurt someone's feelings.

Is it possible to reject someone without consequences? Even if you are polite, the other person will have negative feelings after getting a "no" as an answer. But, here is the thing: if I accept something that I don't want to do, I'll be resentful for it, and I'll be the one who carries those negative feelings in the long run.

I can't say "yes" to everything only to please other people; I should avoid being the "Yes Man."

On the other hand, I don't want people to think I'm rude or selfish. But, should I care about what other people are thinking about me? If they see me as rude or selfish, so what?

If the person who asks for something can't understand that I might unable to do the favor, does this person convey to me?

If someone asks for a favor and expects that I should do it no matter what or without proposing a win-win situation, he/she is the one being selfish and inconsiderate.