If you honestly aspire to learn a subject, take notes on anything and everything you want to retain about the subject. Block off a period of time every six months to review and process all of your notes. Take notes on the notes. You will be surprised how much more you walk away with.
I take note of any piece of information I may want to draw from in the future – newspaper articles, blog posts, books, lectures, conversations, or films. In the digital equivalent, I have binders full of random insights. While I retain some lessons learned, most disappear into the depths of my computer. Only when I review, organize, and annotate my notes months later do the breadth of lessons really stick. Essentially, I take another full set of notes on top of the old to boil them down again into relevant takeaway kernels. The result is a compounded understanding of the material at hand, a level of review I cannot recommend highly enough.