There are different types of memory, visual, auditive, muscular, analytical.. Everyone tends to favour naturally at least one of those, and I think the key is "complementing" with the others.
Muscular memory is usually the easiest to "absorb", but in my experience the most unreliable under stress circumstances. Therefore, spending time working on the architecture of the score, analysing it, explaining it to yourself will help building a more solid foundation in your memory.
I've always thought I struggled with memorizing, but the problem was that I actually set myself very strict rules, as in "memorize all this in a day", and that pressure was handicapping my learning process. Now I let go, and there is no problem at all! I have a very good visual memory, so I tend to see the score in front of me, I practice a lot hands separate to really know all voices, and specially LISTEN to the left hand. And I always work on it's structure, analysing the piece, harmony etc.
Also, the more you memorize the easier it will get, contrary to the belief of many people, this is scientifically proved! Be patient with yourself and enjoy playing
