Many folks believe that MCAT practice exams are perhaps the most important part of preparation. The usefulness of the practice exams is twofold; firstly, the exams constitute an excellent studying opportunity, requiring hours of sustained concentration and the application the important basic science MCAT concepts. This helps to drill the material into your head. Secondly, and more importantly, the practice exams simulate the environment of the actual MCAT exam. When writing practice exams, you should strictly observe real testing conditions. This means not pausing the test when it suits you, only using the bathrooms during actual exam break time and studying in a reasonably quiet place where you will not be distracted by friends or family. You need to sustain the examination mindset for the entire duration of the test.
One of the hardest aspects of the MCAT is being prepared for the pace of the exam and knowing how to deal with the frustrating situation where you are unsure about an answer or confused by a passage. As aforementioned, running out of time on the actual MCAT can drastically affect your score. Being prepared for the timing of the exam is just as important as understanding the material required on the exam. It is worth noting that by mastering the concepts of the MCAT, you will be able to more quickly come to the answers of complex questions and you may even be able to finish the science sections of the MCAT with time to spare, which is useful to go back and check anything you were unsure of (although, of course, it remains unlikely that one would typically have extraneous time in the verbal reasoning section).
The AAMC offers official MCAT practice exams which provide a realistic example of the difficulty and the nature of the real MCAT exam questions. We strongly recommend paying for these exams and writing all of them (or as many as you find sufficient) in the fashion described above, as it will be among the most useful study hours you spend preparing for the MCAT. There are also Kaplan practice MCAT exams available which are reputed to be more difficult than the AAMC practice exams and the real MCAT. Some students like the idea of being challenged by the practice exams so that they excel on the actual exam.