Begin by reading the poem out loud multiple times. Reading it repeatedly will help you understand its pace and cadence. Record the poem onto a tape or through your computer, so you can listen to yourself reading it while you're doing other activities, like driving in the car. Even if you aren't actively committing the words to memory, just hearing the poem will help you memorize it and improve your understanding of the poem's structure.
Dissect the poem so that you understand what it is saying. Picture each line in your mind as an image, and understand how each line relates to the entire poem. Print out the poem, and cover all of the lines with another sheet of paper except for the first line. Repeat that line until you feel comfortable reciting it by heart. Then move the sheet of paper down one line so that the first two lines of the poems are exposed. Memorize the first two lines until you feel comfortable, and do the same thing with the third line and fourth line, until you have memorized the entire poem.
Find a partner. He or she can prompt you if you forget a line, and it will give you practice performing in front of other people. Ask your partner to read aloud every other line or every three lines, while you recite the remaining lines. Keep a copy of the poem near you at all times, so you can glance at it if you can't remember a line. Only use the copy when you get stuck, though, or you will end up relying on it and not memorizing the poem as quickly.