Somehow the writing gets done while you are doing something else—walking to the grocery store (or taking, as Ange describes here, a long short-cut), or riding the tram, or washing dishes, rocking a baby during the night feedings, watching children at the park, hanging out the clothes, cooking supper. When you get a spare moment, you jot it down. I am somewhat mystified by correspondences with poets, perhaps fresh out of an MFA program, who have no job or children, and claim they need to come to Greece for a year, preferably on an island, to have “time to write.” Don’t they have the same twenty-four hour days where they live?
During crazy periods in my life, when I don't have time to sit and try to draft an entire poem at once, I compose line by line as I am doing something else. For instance, driving to pick my daughter up from preschool is a good chance for me to be alone with my thoughts. I inherited the handheld recorder my husband no longer needed after he got his MBA and I keep it next to me in the car. When I get a line of a poem in my head, I grab the recorder. Sometimes I will revise a single line several times over the course of my drive, speaking each separately into the recorder and deciding later which version I like best.
If you don't have a little handheld recorder, I strongly recommend it!