I play clarinet, and I recently learned "Strayaway Child" (a four-part jig) in about six hours over the course of about two days.

One thing I do is just is practice singing, playing, etc. one song at a time until I get it right. Then, whenever I have time, I just sit and practice several songs that I know to refresh my memory. One thing you should never, ever do is throw away your lyric sheets and/or music away. You never know when some sort of selective amnesia will come up. Just make sure that only happens during practices. . .

Peace--Keli