Read through the music that you wish to remember on a printed sheet. Play or sing along with a copy of it while it is playing on a CD.
Sing the first two verses and chorus to yourself everyday for a week. Songs typically repeat themselves half way through. If other successive patterns occur to you, then proceed with singing or playing them out loud as far as you can get.
Play the CD and read the music every day of the second week to refresh your memory, and to ensure that your interpretations are the same as the recorded music.
Sing along with the music as you play it on the CD player. Turn off the CD player and try to get through the song without it. If you come to a place where you can't remember, play it on the CD player to refresh your memory.
Imprint the song to your memory by going for a walk or a drive. Focus on the song as you play it in your mind or sing it. If you come to a place that you can't remember, skip to the next place in the song. Your subconscious will pick it up the next time you go through it in your head.
Play through the song with the recorded version on the last day of the two-week period. Fine tune the lyrics by reading them as you play and sing them with the record. Turn off the recording and sing or play the song in it's entirety.