If it's from a memorable phrase, then the phrase has a lot of redundancy and it's hard to estimate the actual entropy. Generating a random phrase and writing it on a slip of paper works for me. Keep the paper in your pocket and refer to it when you need to, instead of trying to memorize it. Once you've typed it into the computer a few times, you remember it automatically. At that point you can swallow the paper or use your favorite alternate secure disposal method ;).