How to say "No" and feel good about it
It's easy to say yes by default. Sometimes because we don't know how much you've already committed to. If you knew the full extent of everything that you've already got on your plate saying no would become your new norm.
Sometimes you say yes because you don't have a good way to say no. This is the single best response I've come across for saying no gracefully.
"Your project deserves more time, energy and focus than I'm able to give it now"
One for your boss:
"Yes, I could do this for you today. But that means that this, this and this aren't going to happen on time. Are you ok with that?"
Just because a project has appeared on your radar doesn't automatically mean that you should start working on it straight away.
"People think focus means saying yes to the thing you've got to focus on. But that's not what it means at all. It means saying no to the hundred other good ideas that there are. You have to pick carefully. I'm actually as proud of the things we haven't done as the things I have done."
Here are some selected questions to ask yourself before saying yes to something new. You might only need to ask yourself one or two of these before you decide that this isn't something you should be investing your time in:
Does this need to be done at all? (By me or anyone else?)
If I take on this new project, what am I going to drop or put on hold to make way for it?
Is completing this going to move me closer to where I want to be?
Does it compliment what I'm already working on?
Am I the best person to be doing this? Is this part of the high-value core of what I do?
If I wasn't working on this what would I do instead? Which project would be more valuable? (This is an opportunity cost assessment.)
Finally, if you're still stuck for something to say when someone asks you to take on something new that you know that you really shouldn't, consider this your "In case of emergency break glass" reply:
“I must decline, for secret reasons”
E.B. White (via "Keep Going" by Austin Kleon)