Memorising the score away from the piano

In my teenage years, when I first became extremely serious about playing the piano, I was "raised" and trained on the Leimer/Gieseking method, laid out in the two slim volumes (in German) "Modernes Klavierspiel" and "Technik, Dynamik, Pedal". In English, they are available as one volume under the title "Piano Technique" (Walter Gieseking, Karl Leimer). Very strict, very German, more than just a little bit dogmatic, but their approach certainly is solid and has always worked well for me.  I will say, however, that now that I'm in my mid-50s, memorizing a piano score is incredibly hard, exhausting work under any circumstance, no matter what system I use.