Keep a notebook full of metaphors. Write down words or phrases that you feel may be used to describe words or phrases in your poems. You don't need to use them when you create them. The goal is to have enough original metaphors on hand that you always have something you can turn to. There will be times you might use a metaphor that has been used many times before you, but the more original you can be, the more authentic your metaphors will feel. A common metaphor is to write about life as if it were a game of chess. Take a worn metaphor such as this one and think of something else that can be used for a metaphor in the same way as the game of chess.
Examine the subject of the poem you want to write. Determine what the key focus of the poem is and the message you want to get across. Is the poem about love, and if so, what aspect of love are you dealing with? If your theme is the perils of love, you might use a snake to represent love. An example:
It slithers through your heart
When you least expect
And coils in the grass
For the venomous attack
This entire stanza is metaphorical. In the stanza, the snake represents love. The entire metaphor details how one can fall in love before she know what has hit her. Once you get the hang of it, metaphors aren't so difficult to write.
Avoid mixing metaphors. It's bad form. A mixed metaphor is simply two metaphors used in the same sentence. These metaphors can often contradict one another. A mixed metaphor might combine two cliche metaphors, such as "too many cooks in the kitchen isn't a walk in the park." Comparing a walk in the park and cooks in the kitchen is a stretch, and one you never want to put on your readers. Besides, too many metaphors in one poem is a minefield.
Choose only the elements of the metaphor you want to use that enhance the image or message you want to get across. Shakespeare's metaphor, "all the world's a stage," works so well because it accurately describes this wide world where many small plays (everyday life) are enacted.