Many people don't realize that it is perfectly valid to say no to something just because you don't want to do it. That's why often when they say no, they come up with a silly excuse, but I think it is more honest and less rude to just say "no, I can't because I don't want to" rather than coming up with an excuse why not (well, I tend to also say sorry and sound regretful, because I am sorry I am not going to help them, but not sorry enough to do so). Usually, people would understand.

On the other hand, you can do someone a favour and help them out, even if you'd rather do something else, knowing, that if the situation is reversed, they would do the same for you. This principle may seem cold and calculating without explaining all the nuances, but I think that a healthy relationship is based on this kind of reciprocity. I would help my friends and trust them to help me when I am in need.