I use a voice recorder... I never planned on being able to re-listen to every lecture, but use it when I get to a point in my notes that made sense when the professor was explaining it, but looks completely foreign as I'm reviewing. I also use it for take-home exams (open-book, open-note exams, of course), since profs seem to love throwing out examples during class that they then use for the exams. It also really helps kick-start studying if I'm having a hard time getting focused before an exam... It's a little more passive to just start listening to a lecture while I surf the internet than it is to actually crack open my notes 🙂