[Procyona]

When you don't want to do a task you are offered, how to say no to your boss, a coworker or a friend?

I think, a good approach is always to thank for the offer. Then you say the truth, that the action does not go together with your interests. In the end you can give a tip who to ask next, or offer your help in a way that suits you both. This way they are more likely to approach and work with you again.   

[vmashchenko]

It's depends on whom you asks you! You can not say to your boss - no, a couple of no and employee can be fired   

[Procyona]

Thanks vmashchenko. I think if you don't want to work for your boss, you might be in the wrong place.

But, I think it is best for the working commune to be flexible. When you are allowed some freedom over what you do, you become more creative and inspired about the work.   

[Ruslan Nikitenko]

Greetings!
I totally agreed with Procyona regarding flexibility. It is important not just in case of "saying "no"" but in different situations.
"Be water my friend" - Bruce Lee said one day

But for me personally it is quite hard to be flexible. I prefer to say "no" when I don`t agree with somebody or situation. To be honest with myself - it is important too   

[Юрий Николаевич]

  

[Роман Мельник]

If asker isn't right it will accept 'no' well.   

[Gra-ach]

Actually I think it's quite the opposite. If the person who asks you to do something isn't right then (s)he is more likely to get offended or irritated with you if you say no.