Plain-language poems are easiest. Say it honestly, say it in verse, say it without obvious contrivances of rhyme or style and you've done well. But once you enter metaphor-land, well, it's a bit like pulp science fiction. Anything is possible, but not everything is advisable. "You have eyes like vampire fangs," I once wrote of a man. It was true, but a bit lurid, and the poem it was part of had every pitfall of free verse, from ramblingness to, yes, metaphors shoved in purely for the purpose of reminding you that this is a poem rather than just some stuff I felt like getting off my chest.
In improv there's this idea known as the absurdity curve. Those new to improv -- the brave sort, rather than the ones who start off hiding in a corner -- occasionally enter a scene and jump straight off the wall:
"Hello, Joe. Here, help me move this crate."
"Oh, no! An octopus just fell on my head!"
Now don't get me wrong, this can be a great way to approach improv when you're new to it. Just jump out there and say whatever and don't be afraid to look silly. However, as you get slightly better at it, it's as well to develop a little more finesse. The idea of a 'rising absurdity curve' is that you start a scene with the small and ordinary. If you do introduce anything remarkable at the beginning, you take the time to establish it. But sudden dramatic events do not happen until later in the scene, as you reach the climax, at which point elements of the story that seemed normal earlier can and do blossom into full-blown absurdities.
Poetry doesn't have a set 'curve' of the sort that improvisers are taught to consider. Nonetheless, the effectiveness of a vivid metaphor really does depend on context. A poem might go with the improv curve, starting with the ordinary and deepening into metaphor as it draws you in. If you do start with a strong metaphor, you might need to broaden and establish it to make it seem at home. And, as I said at the beginning, sometimes you'd do just as well to leave the metaphors out altogether.
So anyway, I'm fiddling with a memory that I'd love to put into poetry. I write
I never saw a man so golden
as you were, lying by my side.
It's a shoddy approximation of what I felt, but the tone is right. I can't really go anywhere with it, though. I'm writing about something I don't understand. I don't have enough angles. Reluctantly, I give up on describing the exact feeling and decide perhaps I'll just put a little of that in a poem that includes some other stuff.
Late one night, when I'm supposed to be going to sleep, I hammer out a couple of lines that capture so much more of it.
The dawn that rose when I awoke tonight
was only in the halo of your hair.
I can't abandon those lines. They work. It's just that they set a level of metaphor that's going to be jolly hard to keep up with sensibly. This isn't going to be a plain-language poem. Look out, darlin', you're in the Land of High Metaphor. Whatcha gonna do to continue that? Bring out the octopi?
I've started in High Metaphor and now I need substance. Lots and lots of substance, because metaphor, if done well, can eat up substance like nothing else. It's a powerful and dense way of expressing things. One of the reasons I'm finding this so hard to write is that I'm expressing something remarkable that I haven't felt before. It's in the 'Whisky Tango Foxtrot' subgenre of love poetry. However, there have been several times in my life when I've felt something remarkable that I haven't felt before, so I have a better handle on that part of it than on the feeling itself. That helps. I might be able to use that in the poem, but, of course, this now means I'm negotiating two dangers. On the one hand we have Scylla the octopus. On the other hand we have Charybdis, the never ending whirlpool which consists of saying things in a poem like "I don't know how to say it" or "words cannot express this". If words can't express it, why are you trying, dude? Give up and start writing drippy pop songs instead.
It's been a few months, now, but so far I've been able to build this:
The dawn was rising when I woke, tonight,
but only in the halo of your hair,
and I, bemused, perceiving by its light
a whole horizon waiting for me there,
say nothing. I am waiting for a phrase
to catch some faithful gleam inside the haze.
If I could always have a minute more
to stay within the compass of your hand,
then by your touch and mine I could explore
the whole of you and I, and understand
the half-remembered dreams that shimmer through
this little world that takes its light from you.