I presume you are referring to books that you wish to learn from. I used to read fast and retain very little. So I have conditioned myself to slow down, take

I presume you are referring to books that you wish to learn from. I used to read fast and retain very little. So I have conditioned myself to slow down, take notes, but more importantly act on each bit of information. For example, if I come across a new concept I will write a program and "play" with it. By implementing the concept I gain a deeper insight. By trying out different scenarios I get a grasp on the limitations, etc. Then as I progress with the book, the try-outs become more and more complex. My techniques have helped me with programming, networking, electronics books. But even with mathematics. There are many concepts which only became clear when I wrote simple programs to try to use them. Sometimes getting a good visualisation took lots of time to get right, but in doing so I ended up with a better mental model.