Quite often, we'll kid ourselves about the real scope of something we agree to. It's critical to get clear on the real scope of what you're about to take on. How much work will it really take? How much time will that work really take?
And keep in mind Hofstadter's Law which says things always take longer than you expect. It's the accumulation of "one more thing" that adds up and quietly becomes overwhelming.

"What's really involved here?"

There will be a cost to everything additional you say "yes" to. It might be inconsequential, or not. Just be informed. 
What has to give to say "yes" to the new thing? What new skills, resources, or assistance will you have to acquire? How much attention and energy gets diverted from something else, and is that something else more important?

"What's the cost of saying 'yes'?"

Be cognizant of what work simply must not end up on your plate and why. Remembering the why is the key here --recall the emotions, pain, and price associated with taking on the kind of thing you said you wouldn't.
Even worse than taking on work that might not fit with your overall mission is work that you pointedly told yourself you wouldn't get sucked into, but find yourself getting sucked into.

"Is this on my 'to-don't' list?"

We often say "yes" just because it's so much easier than saying "no." It's human nature. 
So capitalize on that tendency by asking yourself if there's a different "yes" you can give so you can maintain the spirit of the affirmative.

"Can I give a different 'yes'?"

What's your purpose and mission? While not every single thing you take on must flow into your mission, the vast majority of your work portfolio should all complement or support that cause.
Asking "What's the point?" brings you back to what the completion of that work serves. If it doesn't matter enough, or at all, you know what to do.

"Will taking this on serve my mission?"

Being inundated and exhausted became like a badge of honor at work**, a way to compare to others to convince ourselves that we're doing all we can with the time we have.
All too often, we see this state as something that's happening to us. More priorities at work, do more with less, have to keep up with our co-workers, and keep pace at home and in the community. But the more we take on the less we actually accomplish at work (and in life).
Being discerning is the skill to develop if you hope to have any sense of balance in your life.

"Can I give a different 'yes'?"

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