On learning the Beethoven Sonatas , pianist Per Tengstrand has some excellent tips. Here’s one that I absolutely agree with. So much easier said than done, but I did spend the time to memorize the Waldstein Sonata (two measures at a time) before I could really play it and I never regretted it…it was the only piece that never had a memory slip in performance. Tengstrand says….
Once opening the score, start memorizing immediately. It is not fun, it’s actually very tedious, but it will force you to analyze the material, it will make you memorize it without using any muscle memory (and that possibility will not come back!) which in turn will be your real, so-called “back-up memory” when the stress in a performance might make your brain’s signals to the fingers go haywire. (read more)