I've also thought about this a lot.
My conclusions, in summary, are that the best way to know more stuff over the long term is reading ebooks and the internet and putting everything useful into an SRS.
Cornell notes, shorthand, mind-mapping, livescribe: who cares? You don't need to take notes. Any method of learning that involves writing down what someone says they read (it's called "university") is ridiculous. Almost anything that can be taught by a person can be taught better by a book.
Speed-reading: perhaps, I haven't done much research. Honestly, gathering new info is a tiny subset of the whole "learning stuff" problem. It's the easy part. The hard part is keeping it in memory, for which: SRS.
Mnemonics: this may be worth learning. However, most things which are worth learning can either be learned without mnemonics or only require extremely basic mnemonics (namely, linking / association). The stuff where mnemonics really shines, like memorizing long strings of numbers of decks of playing cards, is pretty much useless in real life. The stuff that's useful is textured data, data that's coherent and meaningful enough to not need mnemonic techniques (apart from simple linking/association), for instance math or programming or physics. You don't need mnemonics to learn the pumping lemma, but you do need an SRS.
One thing I've been experimenting with recently is to use SRS creatively. I add my own ideas as cards to an SRS system, and just read them every time they're due for review (failing them if the idea seems unfamiliar). Another thing I'm trying is to use SRS to re-expose me to things I've read. If you see a nice LW article, just up and copy the whole thing onto an SRS card, then just look over it when it's due for review. You can add notes to each card about how much time you should spend, like whether you should just skim over the article or read it in detail. If you're reading an ebook, just copy each section into your SRS as a re-exposure card (in addition to adding all useful facts in as active-recall cards).
Seriously, SRS with ebooks just completely subsumes all other techniques for learning stuff.
This comment gave me an interesting idea that I shall try next semester, so thank you!
In every lecture/class/seminar one attends, put exactly 3 things from that lecture into an SRS deck, and then that should cover almost all the revision required.
(The write-3-things-down idea comes from another person... possibly a famous mathematician, does anyone know who?)
Recently, I've collapsed (ascended?) down/up a meta-learning death spiral -- doing a lot less of reading actual informative content, than figuring out how to manage and acquire such content (as well as completely ignoring the antidote). In other words, I've been taking notes on taking notes. And now, I'm looking for your notes on notes for notes.
What kind of scientific knowledge, techniques, and resources do we have right now in the way of information management? How would one efficiently extract useful information possible out of a single pass of the source? The second pass?
The answers may depend on the media, and the media might not be readily apparent. Example: Edward Boyden, Assistant Professor at the MIT Media Lab, recommends recording in a notebook every conversation you ever have with other people. And how do you prepare yourself for the serendipity of a walk downtown? I know I'm more likely to regret not having a notebook on hand than spending the time to bring one along.
I'll conglomerate what I remember seeing on the N-Back Mailing List and in general: I sincerely apologize for my lack of citation.
Notes
(I've also heard a handful of individuals claim that SRS has helped them "internalize" certain behaviors, or maybe patterns of thought, like Non-Violent Comunication or Bayes Theorem... any takers on this?)