I talk to myself. Seriously. I explain, out loud, whatever is on my mind as if I'm talking to another person. I then, also out loud, make objections and ask pointed ?'s as if I were in a lecture or at a presentation. It's a sort of Socratic monologue, and I've been doing it for around a year. Best thing I've ever done with my commute time.

Call me crazy but I find that taking the effort to verbalize my thoughts on a subject really solidifies my understanding.