Back in the day when I was studying at uni, I would carry with me little notes on anything that I would happen to be studying at the moment. Little summaries of what I was studying such that I didn’t have to open a book and read about it in case I just wanted to refresh my memory. These days, with the overflow of information about anything compter-related I attack the problem in a similar way, although in a more techie way: I usually keep text files with important information about the matter.