The fastest way for me is to start with the last verse, sing it a few times, then the second to last, sing through a few times, and so on.

Having tried various ways over time that is the fastest and it seems to result in fewer hitches in the remembering of lines.

Once you have it right once, sing it through from time to time over the next couple of days and it should be there when you need it.

If you find you begin to forget - try fish oil capsules. I kid you not, they have done wonders for my memory once I started to remember to take them of course.