Jason Fried of 37signals:

Attention is about focus and careful, thoughtful consideration. Unlike time – which can be broken into convenient chunks of 15 minutes – attention doesn’t divide quite so neatly or easily.

Next time you say yes to something, ask yourself if yes means “yes, I can do that” or “yes, I can spend my attention on that.” If you’re not willing to spend your attention on it, is it worth doing? Maybe, maybe not, but it’s a good thing to think about the next time you take something on.