The Art of saying "NO"

What is the most difficult thing to say to others?
You all may have different answers, but for me its the simple word - "No". I really find it hard to turn down requests form others... not only friends but anyone - even if i barely know them. There were several instances where i had to sacrifice my own decisions for the sake of others.
It is well and good if its been done for someone care for.. like your friends - its just part of friendship.
But the reality is I just cannot say No to anyone.

I don't really know why, and never had given it a thought until recently. Is it because I want to please everyone around me (...?) or may be to make them feel them that i care for them... i really dont have a clue! Its ok for friends But why do i do it even for strangers?
I asked a few of my friends about this. And the reply i got was very interesting. Some of them said that they turned down the requests by explaining their condition... and the reply i got from some others was really interesting. If they had some difficulty in doing what they were asked to they just found someone who could in fact do that for them. This was a regular practice in my college... It is common practice to sometimes ask others to draw some graph or diagram for us. So if they are busy with something else, they just find someone else who is free or unfortunate enough to make their entrance to the class at that time.

But i never managed to find someone with my condition... at least in real life. If you are a Simpsons fan you might know Marge - Homer's wife. She had this same condition ( in one episode) she just couldn't say No to anyone, but somehow manages to get normal by the end. It was actually after watching this episode was i began to think about my condition.
I dont even know whether its a good habit or a bad one.

Will i be able to get back to normal... Or do i have to?