I find that listening to them in the car over and over again when I'm driving alone - and then turning the CD off and trying to sing what I can remember - and then listening again etc. etc. - helps. If I'm making my own arrangement and cherry-picking verses from here, there and everywhere (as I often am with old ballads) I try to make a rough recording of myself and then take it to the car to listen to and sing along with. Usually an unaccompanied recording, as I like to learn the song first and then let it tell me how it wants to be accompanied.

I do several long (approximately fifteen-verse) ballads and find that if you pick them well and give a bit of introduction, a lot of people are surprisingly receptive to them.