Highlight or underline important phrases. You don't have to highlight every single word in the sentence, just the main information. Avoid highlighting too many sentences in a row as well. Just highlight the major point in each paragraph, and any minor points you think are especially important.
Write notes in the margins. Rephrase sentences and ideas in your own words, or make a connection with an earlier idea. Don't just copy the original words.
Use sticky notes when studying from a book. You can write comments on sticky notes if you don't want to mark up your book. You can also bookmark important pages with sticky notes.
Summarize the major and minor points of each chapter or section on a separate piece of paper. Rephrasing them in your own words will help you remember them better. Keep the points in order and label them by chapter or section, or use titles to help you remember what they are about. Include the page number in case you need to refer back to the original text when you are studying.
Come up with relevant questions that probe the ideas in the text. If you are studying for a literature, history or social science exam, simply being able to regurgitate the information from the textbook may not be enough. You may have to write an essay about or response to the study material based on your own ideas or opinion. You can prepare for this in advance by developing open-ended questions and then answering them.
Reread all of your notes, sticky notes, comments and highlighted phrases. Now that you have combed your text for all the major ideas, you can concentrate on the important parts of the material, thereby studying more efficiently.