Is that so hard?
It is interesting why people can't say "NO", it is interesting to realize those reasons that prevent people refuse requests that take too much of their time. This is a simple word of only two letters and it is not that hard to say this word. So, why it is that hard for people to say that important "NO"? There should be some reasons for it and there are those reasons.
See, there are people who care greatly about opinion of other people about them and so they do a lot of things in order not to offence people or they don't do a number of things for the same purpose. These people have to realize that it is impossible to be kind for everyone. It is impossible to act the way that will make all the people love you.
Those people who like you will like you anyway and those people who just care about you don't will treat you the same way no matter what you do. There is no need in being used by people in case you want them to appreciate and respect you. Moreover, those who use you, who make a fool of you, will never respect you. No one respects those who are of no character.
There are only two letters and it is not that hard to pronounce those letters as a word. Moreover, it is important to pronounce those letters. It is not that hard to say "NO", at least it will take less time of you to say "NO" than to do that whole job people want you to do. Say "NO" and things will be easier for you.