You may not be able to decide to write a poem, but there are a variety of things that you can do to increase the chance of being in a poem-writing state of mind.

Erato, Muse of Poetry, by Sir Edward John Poynter 1870
The first, of course, is to read poetry. We are all influenced by what we are seeing and hearing. Our voices and accents shift towards those we’re talking with, the tunes we hum or whistle are influenced by what we’ve been listening to, and how we write is influenced by what we’ve been reading. Read poetry, especially rhymed and metered, and you’ll be more likely to find your unstructured thoughts expressing themselves in verse.
And that word “unstructured” is also one of the keys. Poetry can come from a chance phrase in your head, from a random rhyme opportunity that you run across that intrigues you for whatever reason, or from an unexpected image or similarity that carries an idea or a metaphor into your thoughts.
So the second thing you can do is, whenever some tiny fragment like this occurs to you, write it down! It is a gift to your conscious poetry-writing mind from your unconscious poetry-dreaming mind.
If it happens in a restaurant, write it down on anything you can take out. If it happens just when you’re going to sleep, sit up, get up, write it down. If it is only a phrase or an idea, write it down. If anything else occurs to you while writing, write it down too. If what you have seems structured, but some other unstructured thoughts are hovering around, write them down too. It doesn’t have to be perfect. You can always come back to it later. But if you don’t grab it when it appears, you are unlikely ever to find it again, or even to remember that there was ever anything there.
The third is to worship the Muse. Really? No, not really, but there’s no other simple way to describe it. There are forces in your subconscious of which you are unaware. They speak to you in dreams when something is really important. You have probably experienced the difference between what can be called “big dreams” and “little dreams”–messages from the unconscious mind, vs tidy-up-and-defrag dreams.
Somewhere inside your mind a creative engine is at work. You can ignore it, and then not only will you never write poetry, but you are likely to screw up your life. Or you can listen to it, let it sing to you, let it give you little gifts of wordplay or insight, and it will help you stay grounded in what is important. So writing down everything interesting that occurs to you out of the blue is a way of honouring that creative engine, that we can call your Muse. Be grateful to it. Accept that your unconscious may know things about you that you have no clue about, everything from how to keep your heart beating, to how to remember the name of someone from 20 years ago, to how to count time and wake you up at a certain time without an alarm clock. Poetry is the tiniest tip of the iceberg of all the unconscious mind can do. Honour it! And it will reward you.
You can find a guide to “Calling the Poem”–how to identify the poetic mood, how to encourage it, how to deal with it–in a chapbook of mine published by Snakeskin, and downloadable for free at http://www.snakeskinpoetry.co.uk/snake236.html
How to write the poem, when you’re in the right mood… More thoughts on that in the next post.