In a short poem, you have to nail the image/metaphor. If you miss, you can't wave your hands ("Look over there—kittens!") or paper it over. There's nowhere to hide in a 5- or 6-line poem. But it also takes a lot of the pressure off, since getting the image right is almost the only thing you can do. There's no complicated structure to manage, no elements or themes to be brought into balance, and questions of cutting an revising are all necessarily at the micro level.
My advisor said the nice thing about short poems is that you can work on them in the car. I smiled and nodded and thought, "You have no idea what my commute is like." But this turns out to be completely true—even on I-93 at 8:00AM. It's possible to hold an entire short poem in my head, revise and rearrange it many times, and still have the whole thing available to write down when I arrive at work. The distraction of having to pay partial attention to the road achieves the same kind of self-hypnotized state I'm always trying to induce when I sit down at my desk: the conscious mind is there, but far enough off to one side that more interesting and unexpected associations can make themselves heard.
The trick is having an image to start from, and since image and metaphor are a focus for me right now this is a particularly useful exercise. I find I look at the world around me more closely, trying every day to find the image I will start from in the car the next morning. This is a good habit for writing any kind of poetry.
Best of all—at least for someone like me who tends to put a lot of pressure on herself over every single poem—this is very a low-stakes endeavor. The poem works or it doesn't, and if it doesn't I can just throw it away. I don't even count the hour lost, since I still got to work on time.