Ask your teacher or professor what is meant by "open book" exam. Are any and all resources allowed? Just class notes? Is there a limit to how many pages of notes you bring? Answering these questions will allow you to prepare efficiently and effectively.
A traditional test requires memorization: What is the student able to take away from the class lectures and study preparation before the exam? However, an open book test is more expansive: How is the student able to take what she has learned and apply it in context? For example, if the test is on psychology, rather than simply defining mental disorders, you may have to identify a disorder based on a case study of a real person.
Because the exam focuses on broad concepts and applying that knowledge, it's best to have your basics covered. Make lists of common terms or dates so that you have them on hand and they are easy to access during the test. Organize your class notes according to topic.
In addition to preparing your class notes, also write down relevant quotes from books or other sources that your teacher will allow. You can use these in exam essays. But be careful to not over-quote; you also need to display your own knowledge of the concepts you've learned. According to Study Guides and Strategies, short quotes can be quite effective, and you should only use a quote if you can't "say it better."