A couple of thoughts:

As an old salesman, I approach the problem like selling myself a song, which means presenting it to all senses (well, not smell) at the same time on the order of 'show them the brochure, and read the words to them while they are reading along with you, so the prospect feels the paper, sees the print and hears the copy at the same time'. Something should get through

I learn songs a lot from records/tapes/performance; if theere's word sheet to read along, that helps, and then writin/typing it and singing it puts the other senses into play.

I also find (as a completely inept banjoist, guitarist/whsitle player, that my fingers will learn things that my mind doesn't know. If I try to recall some song or phrase by figuring it out, it's hopeless, but if I just let my fingers go, somehow the tune is in therepatterned into the nerves.

I do find it's not as easy as it was fifty years ago! I was tickled to hear Pete Seeger on Sunday at the Clearwater Revival have to stop himself into the second verse of one of his own old songs to say, "Oh, wait, I just sang that verse, didn't I?" He didn't have that prob;em when I first heard him fifty years ago, either. And the hills are getting steeper, too.