When writers make a comparison using the words "like" or "as," this constitutes a simile. For example, in his poem "A Red, Red Rose," Robert Burns compares his love to the rose and to a melody. Langston Hughes delves a little deeper in "Harlem" by inquiring how a dream deferred dies, by drying up "like a raisin in the sun" or festering "like a sore." Using similes makes the comparison relatively simple, with generally only one point of comparison: love like a rose in June or a dream dying like a dried up raisin.
Writers use similes as descriptors. For a comparison poem, the writer might observe something and decide to make an unusual association. For example in his poem "Difference" Mark Doty compares jellyfish in a bay to a "school of clouds;" the simile serves as a commentary of the ephemeral nature of the jellyfish. Because similes function as descriptors, the comparison should be clear and create a visual. Focus on what qualities in the original you want to highlight and think of things that also show those qualities. However, avoid clichés because overworked comparisons often fail to create images in the reader's mind but instead serve to distract from the intent.
Metaphors make a deeper comparison. The comparison is stated directly, as with Emily Dickinson's "Hope is a Thing with Feathers." She states directly that hope is the feathered thing and continues the comparison throughout the poem. By doing this, the readers gets a sense of all the ways hope and a feathered thing are the same: eternal, sweet and benevolent. This technique is an extended metaphor. In fact, poet Percy Shelley suggests that creating metaphors inspires new ways of thinking. Metaphors then require looking at the comparison longer.
Writers use metaphors to draw a direct parallel. In "Mother to Son" Langston Hughes uses an extended metaphor so that his narrator can articulate several examples of how hard and rough the stairs of her life have been -- therefore, no set of crystal stairs. To make metaphors powerful, choose a theme in your poetry and maintain it, as in the feathered thing or the crystal stairs. U.S. Poet Laureate Ted Kooser stated that a problem people have with metaphors is failing to sustain them; he recommends writers integrate the comparison throughout. When using metaphor for comparison, work the parallel until it infuses the entire poem.