I'm usually able to say No. I think sometimes that's not even a good thing. I put myself before others, and even though it makes life a little easier for me, I feel like sometimes I need to be nicer to those around me.
I have absolutely no problem what so ever telling someone no. I have supreme confidence in myself coupled with good self esteem. And along with that I really don't give a rats behind what anyone thinks of me. If you believe in yourself and have the conviction that you are right, then telling someone no shouldn't be difficult at all.
I think that we should learn to say 'yes' and help each other more. No one is willing to do anything without getting paid nowadays...
Maybe you just need to get married and have kids. Then you will quickly learn to say no. "Daddy, can I have another cookie?" - "No!" "Daddy, can I play another video game? - "No!" You see how easy it is?
Telling a client NO or, even more powerful, FIRING a client is always difficult. However, it is absolutely necessary to do so when your analyses clearly show that client (or that specific activity for a client) is not profitable. That said, I have taken on less than profitable activities for a client if the overall relationship is profitable. The key is being able to calculate the profitability of a task or a client on a regular basis.
I think Elliot Hulse explains this better than anyone. If you have a moment, listen to him explain "NO" and "I WILL", the two keys to becoming more assertive in life.