Whenever I'm reading things that I want to actually learn and retain, I read with pencil and notebook and write down all the important points in my own words. I've found this to be helpful because it forces me to slow down and think about what I'm reading and how each new piece of information relates to everything that came before it. I've also found that having pencil and paper close at hand encourages picture drawing, which is often helpful when learning something (though it depends on what you're reading).