This is going to sound strange, but a sentence will pop into my head. Depending on what the line is, it might be a poem, it might give me an idea for a story. I write the line down. My poem "Tangibly Invisible," starts with the line, "She cries as she walks."
That line just popped into my head, I had no idea what the poem would be about, but I wrote the line down and out it flowed.