A lot of what I write starts with just a single line, word, or image. With poetry, it's usually an interesting phrase that pops into my head. I like unusual word pairings or images and these can come to me at any time, often when I'm just walking down the street or something. So I write them down and I hold onto the excitement they gave me until I'm able to write.
With my fiction, I start with an idea but a lot of scenes and chapters come to me in the same way poems do.
But I don't wait for inspiration to actually do the writing. I make notes as ideas come to me, but I try to write everyday. A good day for me is getting anywhere from 800-1,200 words down. That can add up to one or two scenes, or a short chapter. At that rate, I know I'll knock out a couple of chapters a week at least.
I try to write one poem a week. So the inspiration can come unexpectedly, but the work needs to be routine, even I don't feel particularly energized or inspired every single day. But that's what the notes are for. I can look back at the ideas that did come as bursts of inspiration and use them as my guide posts.