There are several ways this happens. 1. Lines drop into my mind and some of them, it's clear to me, are the beginning of something interesting. 2. The whole structure of what I want to say comes in a flash and then the real work is finding the images and the verbal music to clothes the structure. 3. Contrarian responses: I read what someone else has written -- "X is purple and hurts like hell." If X has always appeared green to me and has brought me pleasure I feel compelled to write on behalf of X. 4. Place as impetus. Being in a new place makes me want to write. Also when a place makes me nervous, writing helps me cope.