A friend of mine once wrote a blog post called On Reading and Progress that has sadly since been deleted, but I’ve preserved some of it here:
I never read anymore without a pen and highlighter. I highlight passages that stand out and scribble notes in the margins; when I finish a book, I set it aside for a month or two. Then I return to it and transcribe all the highlighted passages and notes into a word document, marking their page numbers. This allows me to review the book and fixes its main concepts in my mind. If I want to review a quote, I can search within the word document for a few words or phrases from the passage, and jump directly to the quote in question, even if I can't remember who wrote it or which book it came from.
I can't stress how much doing exactly this has helped me. I've only done this with maybe 15 books since I started doing it a few months ago, but it's already helped me massively. If you want to do the same, here's some great resources:
These are more for learning on your own time, and if you want something a little more structured, MIT's Open Courseware is awesome as well. They have lecture notes and presentations and recommended textbooks for all the courses that MIT offer, for free. It's fantastic.