One good way to retain information is to create diagrams of what ever you are studying. I learned this from a teacher of philosophy I had in college whose name was Professor Alfonso Verdu. He would draw diagrams of the philosophies that he was teaching. So I took this method as my own, and did diagrams of all the philosophical works I studied over the years. What is amazing is that I can remember diagrams that I did years ago, or those that Professor Alfonso Verdu used in teaching us philosophy. I converted many of my diagrams into digital form in my books and papers. See for instance, The Fragmentation of Being and the Path beyond the Void (http://works.bepress.com/kent_palmer). The key is to avoid using the same format for each diagram like, for instance, MindMaps. Each diagram has to be tailored creatively to the content being portrayed. The work of creating the diagram that is suitable for understanding needs to be kept in a notebook so it can be referenced. If you look at it occasionally when you are thinking about the problems then that reinforced the memory. But just the act of creating the diagrams more or less imprints it permanently on ones memory. Once one has done diagrams like this for a long time, the diagrams are no longer really necessary, but they always help. Not sure why this is so. I guess the brain gets accustomed to think diagrammatically about concepts and one eventually learns just to do it spontaneously.
Here are some examples: