I'll try to help you at this one too. Are you a beginning student? Perhaps teaching yourself? If so, I know exactly how you feel and what you are having trouble with. It's hard to memorize most of the piano literature. But there are ways to make it more effective. It will still take a long time until you gain experience memorizing piano pieces, but opefully this will help:
-Learn one measure at a time
-hands separate practice!
-slow relaxed movements (otherwise you can't speed it up)
-chord approach: take the first group of notes and play them as a chord this will give you a sense about where you will have to move
-lots and lots of patience... you will make mistakes and not pissing yourself off because of them will be something that you must learn
-realize that a piece (for most beginners) takes a long time to memorize and we can only memorize faster if we learn techniques of memorizing
-psychology says that given a short enough segment (a measure) if you repeat it approx. 7 times and then can play it without the score you have memorized it, now you have to polish it
-once again it is very important to do it hands seperate - hands together means more difficulty than just learning the notes in both hands, it means learning very complex coordination, save that for later when the notes are learned
These tips which I have read in the music library of my college have helped me out a great deal. I have only been playing for 5 months now, and I can play the first page of Rachmaninoff's Prelude in C-sharp minor Op. 3 - something that I thought was waaaay beyond my technical level. You'd be surprised what you can achieve if you are willing to give it a ton of time (I probably gave each measure about 1-2 hours of focused practice and I am still working).
Topic: memorising Bach prelude and fuge