NO is learned. Many of us did not have that luxury.
I start off with a friend who has a problem saying no, and during the conversation I will say pass me that book, they do automatically.
I asked them why they did that, answer, you asked.
I'll say if I asked you to jump off a high cliff would you? (one friend said, well I trust you so I'm sure that it would be safe, lol) it should be no.

Saying no requires babysteps, ask a friend to help you out, take small steps and have them ask to go get them a drink, say no.
Make sure no question is real, only part of the no therapy game.
As you start mastering the word, begin to use it in a higher voice, louder and with more authority.
Begin to explain the reason why you "will not" do something, to justify why you would say it.
example: person who has trouble saying no: I will not let you use my car.
person asking: why
no needer: because you do not respect your own things, I will not have you ruin mine.
another
asker: I really need to borrow ten dollars to buy that shirt I want so much,
No sayer: Well I only have enough money to pay my power bill.
asker: Well you can ask someone for it, I really really really need this shirt!
(a real friend would never even ask such a thing)
no sayer: Look you are being so selfish, you go ask that person you mentioned me borrowing from, and take your little tail there and ask yourself.
this should be the end of that conversation
No only can become familar when we use it in many different ways, and in many different levels of authority.
Hope this help others to learn the game of saying :Hell NO!!!!!!!
took me awhile but I think I am the best no sayer there is now,,,,,,,,,,,,,,,,,,,
It gives me control where before I was the doormat.