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.
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.
Craig Ormiston 
I completely concur. The smartest and most successful and significant people in my life have all molded themselves into diligent note takers. Even though there are so many devices out there to make ‘digital notes’ I find the pen and mini-moleskine notebook to be best. (Plus you don’t have to ‘place them in the off position’ when flying!) Then followed by a few sessions of digesting them into documents on my computer, it helps to seal them in my memory, or expand upon for later use. Thanks Craig. I always gain something here!