What's most interesting when I look back at old text books and reading handouts from junior high is the percentage of the page that I likely had highlighted at the time. In my mind I had highlighted this information thinking it was very important, I highlighted it so I might refer back to it later, unfortunately I had highlighted approximately 80% of the page if not more. By the time high school had come around I had become a habitual "page-painter" which is not a particularly good nickname to have - especially when it came time for tests or papers and I would have to rely on my notes and highlights as a shortcut to helping me remember important points. Then came the dreaded history class freshman year of high school. The teacher had taken an entire period to explain the importance of highlighting just the right information - but to be honest, from there on I was very particular about what it is I highlighted on the page.
From that point on I became much more particular about what it is I highlight on a page - I am sure to highlight important dates, character information, setting and plot information, or if the reading is a research article of some sort I typically highlight ideas that I feel pertain most closely with the title of the article or the particular topic of research. Because I do have reading comprehension difficulties I also make quite a few notes in the margins of my reading, I find it much more easy to relate back to materials I have already read if I leave notes summarizing a particular point in the margin.
Lastly I tend to ask myself questions after I have read, many times these questions relate to the important ideas I feel I meant to take from the reading, when I do this if there is a question I cannot answer I likely look back at the notes and highlights I have already made and find what might be missing. Sometimes this step takes a little extra time but it helps me to ensure that I fully comprehend the reading materials I have been presented with.