Try writing a poem in which you describe or tell about something in terms of what it is not. This may be about anything, including a person, a place, a belief, or a relationship.
You may find it helpful to begin by listing images that describe your subject in terms of what it is, then modify those images by converting them to tell what your subject is not.
Read “Disillusionment of Ten O’Clock” by Wallace Stevens.
A variation on this prompt is to write a poem about something you didn't do, a choice you didn't make, a love you left behind. For an example, read Robert Frost's "The Road Not Taken." http://www.poets.org/viewmedia.php/prmMID/15717
Another wonderful "not poem" is the title poem from Penny Harter's book The Night Marsh (WordTech Editions, 2008).
The Night Marsh
By Penny Harter
This is not about frogs,
their rhythmic croaking
from the swampy edges of a pond,
nor is it a landscape
of the hunter and the hunt.
This marsh extends for miles,
water glittering here and there
The sleepers feel their way
as they stumble from wet land to dry.
There is no hand to hold,
no voice dripping like moss
from the trees, singing,
Come this way
into the darker dreaming of the day.
Copyright © 2008 By Penny Harter.
Reprinted by permission of the author.