I agree with Will Fly about writing them out. Just the process can **help** you set them in memory. I do often 'see' them on my memory of the paper.... and the note card trick of just the first couple words of each line to glance at is also good.
   Some people's memory works better differently, so it's all just trying various things. Also some songs are easier because the ARE a story, while others have no orderly procession and are just rote repetition.... and that is best done when you are young... *wry grin*.