There is a bit of a disconnect that PMP Exam takers report as they are preparing for the exam. Because you must be an experienced project manager to take the exam, you bring years of experience in managing projects and using tools & techniques with you. Often, these are based on company internal project management best practices and tactics that you found working for you. However, the PMP Exam requires that you apply the concepts from the PMBOK Guide to real-life situations as presented in the exam questions. If the methodology that you are experienced in using is not aligned with the PMBOK Guide, then you may pick the wrong answers in your test.
Furthermore, the projects you manage may not have required you to deal in all the PMBOK Guide's Knowledge Areas. For instance, risk management was something I did very rarely on my projects and maybe in your career you never had to deal with procurement. So it is likely that you’ll be more comfortable with some project management knowledge areas and processes than others. This can lead to two problems:
First you may feel that because you are an absolute pro in scheduling (after all you have years of experience here) you can slack off in your studies and rely on your own project management experience instead. You tend to minimize studying for the areas you know best. But this can hurt you because the PMBOK’s approach is the correct approach for the PMP exam.
The second is the tendency to minimize the importance of project management areas with which you are unfamiliar. Just because I didn't do much risk management doesn't mean that it isn't important. But we are creatures of habit, so it's only normal to also think that the "unimportant" areas on our projects are also "unimportant" on the exam. PMPs are expected to demonstrate a good understanding of all aspects of project management as defined in the PMBOK. So pay particular attention to the processes with which you are not familiar.
So what's the best approach? I always recommend to my students that they study the PMBOK Guide at least twice before taking the exam and that they immediately start using the practices learned on their projects. Applying the theory from the PMBOK Guide on your projects is the best way of learning it and passing the exam.