Highlighting alone won't do anything, but once you need to review what you've already read, it can be time efficient to have the most important parts highlighted.
Besides, a customized material tends to be memory friendly.
A man builds. A parasite asks 'Where is my share?'
A man creates. A parasite says, 'What will the neighbors think?'
A man invents. A parasite says, 'Watch out, or you might tread on the toes of God... '
The more senses involved, the better. When I want to learn it's definitely seeing it and rewriting/repeating it in words/terms I understand, then repeat. (Sight/Sound/Touch). I definitely need to re-visit as well if it is going to stick. I do this alot when I do public speaking. With this method it becomes less about memorizing what exactly I'm going to say but just expanding on whatever point I'm trying to make. Then you can add lib or whatever in the moment.
edit- for the purpose of test taking- practice exams work, along with what's up there. If
People who highlight:
How do you know that highlighting actually improved your learning?
"I did just fine!" isn't really a proof, since you can't know if you would have done better if you hadn't spend all your time highlighting.
I for one never highlighted anything. When taking notes (in certain stages of school these were mandatory, and even graded at the end of course. ), I never used anything else than a pencil. My handwriting was so awful that most of the time I couldn't figure out what it says myself! Until final years of college, I never read for exams (okay, once I did and got a bad grade). And in college I mostly read because I tended skipping the lectures (all the slides are available online).
All in all, I was very lazy student. But I kept getting good grades because I paid attention during the classes. Writing notes just distracted me (or more precisely, gave me no time to process the data... I just automatically write what I heard).
Then again, I guess primary Si helps a great deal.
I highlighted and underlined for a while before I realized that I had started to underline 1/2 of the books I read... and that I may as well re-read it... so that was pointless... but writing down main points, themes and referencing important passages as I was reading - at decent stopping points - really really helps me retain info... but what really solidifies a book's contents (for me, at least) is going back and trying to summarize the book and its main themes... actually writing a mini-paper... maybe a page or less... highlighting didn't help me at all with this, but the notes I took during reading delivered everything I needed... this forces me to think about what the book was actually about, not just regurgitate little bits of it that seemed important while reading... and now I read books with the idea of writing this mini-paper, which has gradually taught me to read books in a different way... and I find I'm retaining material much better... and, if I do forget something, I go back and read what I wrote and usually it pops right back into memory...
...wow... typing all this out and reading it back makes me feel a little insane... whoa...
I highlight and make my own mindmaps. I'm a visual lerner and highlights and mindmaps help me a lot. Highlights make the words stick when I look at them a bit longer and then maybe I get back to those later without having to read all the material, just the important parts. Mindmaps are created while I analyse my own thoughts. When I see how it relates, I understand and thus also remember it better. Little details won't help if the big picture is not understood so I always concentrate on big themes and connections and ignore little details but miraculously sometimes can recall those too. Oh, and of course I always got/get top grades!