Look around the text before you start reading! You've probably engaged in one version of previewing in the past, when you've tried to determine how long an assigned reading is (and how time it will require to read it.) But you can learn a great deal more about the organization and purpose of a text by taking note of features other than its length.
Previewing enables you to develop a set of expectations about the scope and aim of the text. These very preliminary impressions offer you a way to focus your reading.