I always memorize the pieces I really want to learn. In fact, I could say I memorize everything, since I don't learn/play pieces I don't like. Though, sometimes I got so bored of a piece, that I just didn't play it anymore and cause of that, I just forget those pieces. I play the pieces, wich I really like to play, each day 1 or 2 times, so I keep them fresh to perform them.
I memorize pieces bij visual and hearing. When I learn a new piece, I learn each single bar. I look at the sheet music, so I know what to play and then I just look at the keyboard, remember the notes/handpositions and then listen carefully to it. I do this cause I'm probably one of the worst sightreaders on this forum.

.... Though, my memory is quite good and I memorize very quick, so that's an advantage I have.
Also, another reason why I memorize pieces: if you go to someone who owns a piano and people are asking you to play something, but you don't have scores with you, so you can't play the piano... For example. I want to avoid that no matter what. And the hard work pays off; often people are impressed if you play something by memory. People who aren't "into" music (a lot of people), often think that musicians need sheet music to play something. But you're in front of that piano, without sheet music, but still playing the cool tunes on the piano... I noticed people really like to see that and are quite often impressed.
