Look briefly over the entire book or article to get a feel for its structure and how its argument or arguments will proceed.
Pay particular attention to introductory and concluding paragraphs. These often contain summaries of important points.
Look for certain words and phrases that can tip you off that something important is coming up, such as "In sum," "The point is," "Most importantly," and so on.
Consider reading the conclusion first. It's like doing a maze backwards: If you know where you're trying to end up, you can find and understand the path better.
Look back over the book or article the next day, reading only the highlighted material. Do so again in about a week. This will help the material stick better in your mind.
Remember that this is a skill: Be patient with yourself if you're having difficulty with it. Practice makes perfect.