Singers often have to work from memory, in opera, for example. So, ABRSM singing exams require all song to be memorised. What is the best way to memorise vocal music? Different things work for different people.
Some of these may work for you.
Think about what comes next. What is the first words of the next verse/phrase?
Tell the story of the song using the nouns and verbs of the text Visualise the actors/objects/verbs in the song
I heard Barbara Bonney say once that she used long journeys to write out the words of songs from memory, several times.
Look especially at phrases that are nearly the same, but not quite, and remember where in the song they come. This is especially true of cadences.
See also this external link: Memorising music