I think there is definitely something to the 'mental recording' idea; I have heard many singers forget their own songs particularly if they have not sung them in a while, but not only do they remember other people's songs more readily, they also seem to remember their own songs which other people sing, or on which people reliably sing along with them. Of course, that may also reflect a bit of the 'remember the memorable' idea, if the songs that other people take up to sing are a writer's best work, but that can't be all; I have heard songwriters forget some truly wonderful works. I wonder if composing on tape and singing along with oneself over and over would make the song stick better than composing/recording the words on paper.