A metaphor can make or break a poem. Metaphors have the power to clarify emotion, appropriate images and build a connection between reader and text. There are many kinds of metaphors: absolute, active, complex, compound, dead, dormant, dying, extended, implicit, mixed, pataphor, root, simple, submerged and synechdochic. Different types of metaphors require different structures. No matter what kind of metaphor you craft, following certain guidelines can help strengthen your chance of wowing your reader and enhancing your poem.
A metaphor is a figure of speech that describes one thing in terms of another. The two items are seemingly different, and the comparison is implicit. Metaphors abound in everyday speech. If you hear someone say, "He was a bear," after dealing with a person in a bad mood, you have heard a metaphor -- you infer that this person was loud, irritable and potentially dangerous.
Because conversations are so littered with metaphor, certain phrases get overused and become cliches. To write an effective metaphor, avoid these phrases. Calling a beautiful woman a flower will not spark your readers' imaginations -- this metaphor has been overused. However, you can rehabilitate this metaphor by adding some surprising element to it. Perhaps you want to be playful and compare this beautiful woman to "flour." Don't be afraid to take risks with your metaphors, and recognize and eliminate phrases so overused that they add no new meaning to your poem.
Of course you are free to explore the implications of unusual images and experiences, such as watching a meteor shower or witnessing a car crash. However, do not ignore the well of material you witness every day. Chances are, your reader may be more familiar with the empty McDonald's bag on the sidewalk than the train veering off the tracks. Why not use the former to describe the feeling after a great disappointment? Explore the implications of things you see: the crowded bus stop, the bored cashier, the squeaky staircase. What emotions do these images provoke? Keep a journal of your observations and return to it when you write.
Ultimately, the best way to judge the quality of your metaphor is to consider how it affects the rest of your poem. A metaphor may seem fine out of context. Consider for example, "Sonnet 18" by William Shakespeare. Comparing the speaker's love to a summer day works very well in this sonnet written to praise and immortalize his object of affection. However, if we drop this metaphor into Sylvia Plath's poem, "Daddy," which details the speaker's dark and painful relationship with her father, then not only would the metaphor seem out of place, but the entire tone of the poem would change. Evaluate your metaphor by asking yourself what you want your poem to convey, and then reading the poem in its entirety to see how the metaphor helps bring your goals to fruition.