Start off with focused practice. Find a quiet spot where you won't be distracted. Be prepared to dedicate some time to learning. Committing a song to memory takes a good deal of concentration.
Read the song's lyrics aloud as if reciting a poem. Repeat this action until you get an idea of how you wish to express the song.
Write the words of the entire song, speaking each word aloud as you write. Do this several times until the song begins to stick. Continue to write out the word but now sing each phrase as you write.
Sing along with your favorite recording of the song again and again.
Rehearse your song a capella in the shower, while driving in your car or on your daily walk. Singing without music challenges your memory to retain the words.
Sing your song everyday until performance time. Skipping a day may induce memory loss, particularly if the song is very new to you.