How good are you in saying "No" politely?

Iam cool and calm person, so iam always soft when i speak. When i have tosay NO, iam always sof and polite. My friends say that iam always polite person, i too believe iam polite always.I hate the people who is harsh.

I am very bad at saying no. It's very hard for me to say no so I usually say yes and find myself in all sorts of weird (or at least weary) situations. And when I do say no I say it in a wavering voice and end up feeling very very bad :( (yes I know, I have to learn to say no without feeling guilty about it) :)