I use a good many of the same techniques as given by several above. After I actually know the song but get sloppy about it is when I do what I call 'bringing it up to my face", in other words, visualizing the sheet that I typed up and singing from it. Seemingly I can literally read it- I even "see" the key scrawled in the corner. It is an active process- at that point I'm not trying to remember the words. Often I'll see the whole problem verse and need to "look" only once.