Here's a direct quote from Bill Manhire about how a poem starts, a question asked at Lucy Dougan's book launch today of herself and Fay Zwicky:
'The great pleasure, for Manhire, comes from how poetry mines the imagination. "One of the truisms of writing is that you have to show a little respect for your own ignorance rather than start off as a know-all . . . writing what you don't know is at least as important as writing what you know. The whole business of surprise as you write is the great joy."
Often he will just sit down and scribble words for several pages, "until suddenly you just bump into this very strange phrase that you couldn't have anticipated and that is charged with all sorts of resonance, so you chase on after what that phrase suggests and suddenly you are in the territory of what you don't know; that unmapped space".
Or he will start with some sort of trigger, "often just the music of a phrase and I will let that generate what it can - cadence, image, mood, tone or voice - before I find my way to meaning. I want to bump into meaning rather than assert it from the very beginning". '
This and more at http://www.theage.com.au/news/books/the-accidental-poet/2008/02/22/1203467295623.html