My father learned a lot of songs over the years, and in order to remember which songs were in his head, so to speak, he typed up a long list and literally papered the back of his guitar with it. I was at a Song Circle one time when one of the performers remarked about his memory and another one picked up his guitar and turned it so the rest could see the long long list. I think he went through them routinely to be sure he did remember them. This is back pre-tablets and such (and he hated the use of Rise Up Singing when people stood and sang out of the book, not knowing the song or making eye contact with the audience. If you didn't know it well enough to sing without the book, you didn't need to be singing it publicly yet.)