Sing it.

That sounds fatuous, but for me it's repetition which is important. Transcribing the words helps, otherwise just listening to a song until its ingrained and then singing it until it's thoroughly embedded does it for me. O go back to the original from time to time as a check, but I don't worry about minor changes to the words unless it's a phrase which is particularly significant.

I try to learn the song as a whole, rather than verse by verse, even if that means there are gaps to begin with. I just note them, go back and fill them in, and try again.