I learned this little trick when I was on tour with my church choir a few (ahem!) years ago, and we had to give hour and half long concerts singing completely from memroy (including selections from Handel's Messiah).

I memorized the first and last word in every line of each song. After a while, they 'trigger' the rest of the line -- and it makes a very quick 'study' of the song in your head right before you go on stage.

-- Áine