Seth Godin nailed it with when to say the ‘evil’ word ‘NO’ in his blog (http://sethgodin.typepad.com/seths_blog/2016/03/on-saying-no.html):
If you’re not proud of it, don’t serve it.
If you can’t do a good job, don’t take it on.
If it’s going to distract you from the work that truly matters, pass.
If you don’t know why they want you to do this, ask.
If you need to hide it from your mom, reconsider.
If it benefits you but not the people you care about, decline.
If you’re going along with the crowd, that’s not enough.
If it creates a habit that costs you in the long run, don’t start.
If it doesn’t move you forward, hesitate then walk away.
Warren Buffett put it in another way: “Being disciplined enough to not do what you don’t want to do, even if everyone around you (and that voice inside your head) is telling you that you should.”
And Godin sum it up nicely:
The short run always seems urgent, and a moment where compromise feels appropriate. But in the long run, it’s the good ‘no’s that we remember.