I read books to get me to think. I often want to refer back “this book had some useful nuggets” but of course can’t locate the passage. To ensure I don’t forget I now force myself to have a pen handy. Spot a useful nugget, put a bracket around it and then also doodle a 1cm line on the outside of the page so that as I’m flicking through the line draws my attention more easily then the underlining (tends to blur into the text).
These days I sometimes even take a photo in the hope one day it will be easily searchable.
Sounds easy because it is. Yet it took me years to take up this simple habit for future me.