How do you learn to say: No {?}

How do you learn to say No?  

For you, this might be easy. 

For me. Not so much.  

Oh yah...
I can say NO to spending money when its not in the budget.  
I can say NO to buying something I don't need when its just a want. {Doesn't mean I want to, but I do know how to do this and can carry it out.}
I can say NO to eating more food when one serving was enough.  Even when I want more.  I can say NO because I don't want to gain more pounds.
I can say NO to getting up early when I want to sleep in.  
I can say NO to a dinner date when I am out of town and realistically can't do it.
I can say NO to something when I am already booked, literally booked, with something else and there is on way to rearrange something and make it work.
I can say NO to eating sugar or wheat because I know how it makes me feel.
I can say NO when I see the end in sight.
I can say NO when I want to being ugly when I want to when I think about how it will hurt someones feelings with what I want to say.
I can say NO I don't want to be sick, but that doesn't mean I won't be anyways.
I can say NO you didn't ask permission.
I can say NO you are incorrect, here is the proof with the documentation to support the case.
I can say NO to many things...but

You might not understand but there are some things I have a hard time saying no to.  Some things that you would think by now I have mastered the ability to say NO to.

I cannot seem to say NO to work, even if it is not paying work.  
I cannot seem to tell clients or friends NO.
I cannot seem to say NO to things that don't let me stop long enough to tare of me.
I cannot seem to say NO when I am tired, utterly exhausted and ready for bed.  
I cannot say NO when I feel obligated to do something that I feel someone requires or expects of me.
I cannot say NO when I know you need the help.
I cannot seem to learn the balance of saying YES and NO.

I ask, because I want to learn.  I ask, because I want to be different.  I ask, because I need help...how do you learn to say NO???  How do you work at doing this when you know you have a problem with doing so?  How do you achieve a balance at saying YES and NO?  How do you say NO when not enough people say YES?  How do you say NO when you heart longs to say NO?  How do you say NO when you need to say NO because its best for you and not feel like you are being selfish?  How do you say NO when you know it will hurt someones feelings?  How do you simplify the thought process behind learning to say NO?