Remembering a song when the lyrics are in that muzzy quantum state, with multiple alternatives for every other line, is a nightmare - fortunately I find that once a song's finished the lines seem to stay put. Singing it in public helps to get everything nailed down (at least until the second time I try to sing it in public).

Singing unaccompanied makes it easier to keep it all in my head - at the moment I'm reasonably confident of 20-30 traditional songs and a similar number of my own, plus various bits of Dylan, Nick Drake, etc. But it is a bit fragile - does a song exist if it's never been sung in public, recorded or written down? (As with one song I wrote a couple of years ago but got tired of almost immediately. It's still taking up headspace, though.)