For the last few years whenever I need to accumulate a large and comprehensive declarative knowledge of some subject I use spaced repetition and Anki. My system is simply find a good several hundred page book on the subject then spend four or five days of intense card making. At the end I have a working knowledge and memory of the subject matter at least as good as what most people accumulate in months of ad hoc reading and studying. Also the mere act of distilling the information to cards helps clarify what's important. To maintain the knowledge, I just keep up with the repetition schedule. If I want to go a bit deeper, I'll find the most tagged questions on Stack Overflow and Anki the good stuff from those.

I've had a few situations where people thought I had years of experience with some technology, framework, or programming language when actually I just picked it up last week. One situation in particular, I had a buddy who was using WordPress as the basis of a pretty profitable affiliate site business he was running until he ended up running into some issues that involved needing to make some pretty significant modifications to WordPress itself. He offered to get me in on his affiliate business if I could solve his problem for him. To make a long story short, though I had never even looked at PHP or WordPress before this, within 4 days of using spaced repetition to learn PHP and the intricacies of the WP framework, I was fluently writing plugins that handily solved his problem and a whole lot more and never had to open the reference books again as the whole thing was committed to memory. Not tooting my horn just giving an example of how what ESR was talking about above can be actualized.