This is from Natalie Goldberg's book "Writing Down the Bones".
Take a poetry book. Open to any page, grab a line, write it down, and continue from there. A friend calls it "writing off the page". If you begin with a great line, it helps because you start right off from a lofty place.
"I will die in Paris, on a rainy day ... It will be a Thursday," by the poet Cesar Vallejo. "I will die on Monday at eleven o'clock, on Friday at three o'clock in South Dakota rising a tractor, in Brooklyn in a delicatessen," on and on. Every time you get stuck, just rewrite the first line and keep going. Rewriting the first line gives you a whole new start and a chance for another direction - "I don't want to die and I don't care if I'm in Paris or Moscow or Youngstown, Ohio".