I don't have a problem with saying no, it is the shortest complete sentence in the english language. I do however have a few conditions and considerations when dealing with peoples requests.
I try to remember that if I want to excell in an environment it should be used sparingly with people of authority. I stress Want because there are some settings, especially social ones, that i simply do not wish to be considered a figure of authority.
Authority is illusory. Responsibility is the real issue there, and I dont want to be responsible for a social group. You are not trying to get a job done, you are trying to create a happy environment, and since people are all made happy in different ways, it is a fools game.
If it is a social situation I use it whenever I feel like it. I am there for enjoyment, not compliance, and while one doesn't rule out the other, I usually wont give up my fun to placate another persons need to control. Some tend to view my lack of compliance as me being controlling, yet that is illusory, short sighted, and narrow minded. If you feel someone is being controlling merely by exerting their own will on their own choices, then you have serious control isssues of your own.
If it is a proffesional sitiuation I use it much more sparingly, remembering, as you did SB, that I profit from saying yes instead; either emediately as in extra pay or in the long term, as in being upwardly mobile and in line for a raise.
I also notice that if asked nicely and respectfully in ANY environment by someone i care for or respect, I will usually aquiesce to any request. If threatened or treated rudely, I will simply not comply. I do not reward abusive, arrogant or demeaning behavior with compliance. It is that simple.
So I guess my answer is that I make those choices on a case by case basis, depending on the environment, the person who makes the request, and the tone and method of the request.