I've ever received about writing was this:
Think of the one thing your protagonist would never, ever do.
Then, make him do it.
On a related note:
Think of the one thing you would never ever do again.
I know what mine is.
1. Ow. They hurt.
2. I couldn't type. TRAGEDY!
3. I woke up in the middle of the night in a panic, dreaming that each of my fingers were in tiny individual jail cells. And ow! (see #1)
Never. Ever. Again.
What's your one thing?