Write down the main subject of your slam poem, such as "decriminalization of drugs" or "the plight of the homeless." This subject will serve as a focus for you as you write.
Write a rough draft of your poem, focusing only on the emotions and ideas you want to express. Do not worry about verses, rhyming or even complete sentences---simply get the feeling you want on paper.
Use slang, but only if it is comfortable and natural for you, otherwise your poem will not sound authentic. Read through your poem and find the action verbs, then ask yourself if there is a stronger verb you can use there instead. For example, instead of "cried," you might use "sobbed" or "bawled."
Look for a few places to use metaphors or similes in your slam poem, which are common elements of any type of poetry and help add description. If one line reads "I felt empty," change it to something like "I felt like a hollow drum," which gives the listeners and readers a more visual sense of your emotions.
Read the poem aloud to yourself, then for a few friends. Read with emotion and passion: slam poets are performers. Find the rhythm and lyricism of your poem, and if some parts are awkward, revise as necessary.