1. Not saying yes.
2. Saying yes to something I'm not passionate about.
Simple as that.
When I have an opportunity before me, regardless of how much fear it strikes in my heart, I listen to my gut. Is this something that excites me? Is this something I would be proud to do? Would it give me valuable experience? Would I look back and wonder "What would have happen had I taken that chance?"
As for the second point - too often I do something out of obligation or in order to please somebody. These are the activities that have me dragging my feet for miles. When you're not passionate about it, just say no. You'll thank yourself for not wasting your time.