Turn the headings and topic sentences into questions in your mind. If, for example, a heading is "weapons used by knights," rephrase it as "what weapons were used by knights?"
Skim the paragraphs for answers to the question you formed in Step 1. Looking for answers to a question rather than just absorbing information makes your reading more focused and increases your ability to comprehend.
Skim the table of contents for the main or relevant ideas, and read headings on their own. This will allow you to discard information that is not important to you and focus on information you need.
Read the introduction and conclusion if you are unsure if something is relevant to you. These brief summaries will quickly let you know if you should keep reading or not.
Avoid highlighting or writing in margins; they will slow you down and focus you on writing and highlighting rather than on reading and comprehending.
Practice these steps on a daily basis. Sit in a well-lit area with the book or article at a 45-degree angle.