Can I Say No?

thinking...
how can you say 'no' to a dear friend who really needs your help but it is for something which you believe is wrong.theoretically, it should be easy but it is not easy if you do not want to hurt their feelings.i am no angel and have gave in so many times to my friend's requests and always felt guilty after that, in spite of being happy to help a friend and in other way myself.
it is always hard to say 'no' when you see the disappointment on their face or hear it from their voice, which is the last thing you want your friend to feel..i don't like feeling torn and choosing between my friend's feelings and what is right.but this time, my choice is clear...i want to be on the right track.