For me, I need to be in the right mood. When I was communting and working regular hours, the journey back home on the train often did it: I was quite tired and my mind was not focusing on immediate tasks of the day (that sensitive meeting at ten), but was reflective. If I say that I wrote some poems in a trance-like state, I don’t mean anything that would have worried my fellow-commuters, though they might have noticed I seemed to be thinking hard. Maybe the travelling helped too: I’ve always been excited by the sense of travel and change, and for me that was associated with a sense of other worlds, of other kinds of existence.
Being tired helps: you are less controlled and controlling.
Music can do it – but for me, only classical orchestral. I’m a great fan of folk rock and Johnny Cash, but it’s Sibelius and Bruckner, Nielsen and Rachmaninov, that take me into a mood where poetry can be created.
The silence of a Quaker meeting can do it, as can silent lone contemplation but perhaps not so well.
Being outdoors in open country can help a poem form, or coming back after such an outing if I have time and space to think about it. Rain on the windows can be magic and the sea always is.