Saying No.

If you are travelling to a city, you implicitly say no to every other city until you reach there. If you keep saying yes to everything that comes up during your journey, you would never reach where you intended to go.

There's a time to pick up projects that come your way and say, "Yes, why not?", especially when you are a beginner looking for experience and opportunity.

But, equally or even more important, there's a time when you say, "This is the direction I am going towards." and say no to everything else.

HT to Derek Sivers for being the flagbearer for "Hell Yeah or No!"