No please :))

I was wondering today how is it that some people are able to say a firm NO to something they dont want. I for one have never been able to do it and so I will be the first to admit that it is indeed an art to say NO. And I dont think I am the only one. People instictively recognize my kind and ask /demand /plead in such a way that they know it will most definately work. The resultant being that I end up doing something which I never wanted to in the first place AND do it as if it was my idea all along. In my defense I try to avoid people whom I know would like to manipulate me. To be very honest, I am thankful to the handful of friends / relatives who KNOW that even though my lips are saying yes sure why not, my mind is probably screaming NO WAYS !! and they let me escape. But how long to go on like this. I have often tried to learn the art of saying NO, even tried it sucessfully, but again anded up feeling awfully guilty that I did so. Refusals- polite or not- hurt - or they hurt me , so I automatically assume that they hurt others to, although people who do say NO assure me that it doesn't. No one is indispensable. If I say no chances are they will find someone who says yes in a matter of minutes if not seconds. So what is it that stops me from stating my mind ? Is it a need to always please everyone? But I do know thats a sure sign of failure for no one can please everyone at any time right? Am i gaining more friends and less respect by being like this? Or is that too a myth that you are respected more if you know how to say NO? Reading this, I am sure you will categorically fall under one group- mine or the elusive, talented, extraordinary of those who can manage a firm NO. If you belong to my group then come lets sit and lament over our weakness (you can't say no to this too- I know !) and if the other then,  please dont smirk at us. You "know" you never "know" when you may need a person who can"no"t say a "no" ! Kudos to us !! :))