In my acting classes the teachers talk about the memory muscle and the need to “work it”. They suggest we memorize a different poem, song, a couple of sentences from the newspaper, and so on, and promise if we did that every day, then memorizing would get easier. And they were right.
I doubt my acting teachers read the book, The Harvard Medical School Guide to Achieving Optimal Memory by Aaron P. Nelson, Ph.D and Susan Gilbert, but that excellent book says the same thing. The brain is like a muscle and needs to be exercised.
The research also shows that physical exercise, such as taking a 45 minute brisk walk at least three times a week, dramatically improves all areas of mental functioning.
So combine the two ideas and memorize something as you walk.
I like to walk with my iPod (or cd or cassette player) and memorize a song by listening to it over and over. Other days I will write out a short monologue, a poem, or a few sentences from a magazine and carry it for reference as I memorize. Try it and I promise your memory will improve.