When I was in sec school, our teachers encouraged us to write our study notes on cards. The cards were around the size of a 3R photograph. They fitted our pockets thus we could study those notes even when we were on the bus. It was easier to take out a stack of cards from the pockets then to take out a stack of A4 papers from the file or bag. Moreover, compared to A4 paper, the cards did not get crumpled easily. And because of the size of the cards, the notes had to be brief. I found this method useful for science equations and diagrams. A lot of time was spent on writing notes on those cards.