Contrasts are a good way to add tension and texture to a poem, by, for example, suggesting opposing emotional states, or clashes of personality, or environments. They can also act as a structuring tool, as the following poem by Jay Leeming illustrates:
The Light Above Cities
Sitting in darkness,
I see how the light of the city
fills the clouds, rosewater light
poured into the sky
like the single body we are. It is the sum
of a million lives; a man drinking beer
beneath a light bulb, a dancer spinning
in a fluorescent room, a girl reading a book
beneath a lamp.
Yet there are others — astronomers,
thieves, lovers — whose work is only done
in darkness. Sometimes
I don't want to show these poems
to anyone, sometimes
I want to remain hidden, deep in the coals
with the one who pulls the stars
through a telescope's glass, the one who listens
for the click of the lock, the one
who kisses softly a woman's eyes.
from Dynamite on a China Plate: Poems
Backwaters Press 2006
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This poem uses light and dark in its two stanzas. It also shows us a whole city (a million lives) in the 1st, and the particular thoughts and desires of one person in the 2nd. It may be that once you use one pair of conrasting images and ideas then others find their way into the work too.
Brainstorm for a list of contrasts and opposites. Here are a few to start you off, but write freely for 5 or 10 minutes and find what matters to you:
wet and dry
mountains and valleys
death and birth
sound and silence
cold and warmth
Work on a draft of a poem that uses two stanzas to explore your opposing ideas.