First, obtain official MCAT practice tests from the AAMC. Take the full practice exam at least 2 months before you take the real MCAT, and compare your practice test score to your target score. Identify the question types that you missed, and make a study plan to review and relearn science material.
Next, start working on practice questions from trusted study guides. Books you can buy in the stores usually have high quality, relevant content because many test prep companies actually buy questions from the MCAT makers. However, there are many online study systems and websites with quality content as well.
Where ever you choose to obtain MCAT study materials, always compare the bilogy and physics material to the questions found in AAMC publications. Use MCAT study aids that have the most exam-like material.
Make sure that the time you spend reviewing a particular concept is proportional to its frequency on the MCAT. For example, there are likely to be more questions on mitosis than meiosis, therefore, if you are rusty on the steps of both processes, spend more time learning about mitosis.
If something is rarely or never tested on the MCAT biological or physical sciences, don't waste any your study time trying to learn it.
Practice reading quickly, and learn to jot down quick notes for the MCAT science passages. Reading can take up a huge chunk of your time. Taking notes helps you avoid having to re-read, and consistent practice helps you do everything faster.
One method is to write a few words of summary for each paragraph, or diagram the flow of argument. Your notes should reflect the structure of the passage at the very least. In case you cannot avoid having to re-read some parts of the MCAT passages, your notes can serve as a rough index to find the info more quickly.
When the clock is ticking, try to resolve tough MCAT science questions by process of elimination. If you can eliminate 1 or 2 choices for sure, and have good gut feeling about one of the choices, just make your best guess and move on.
Practice relaxation techniques, and take measures to reduce your test anxiety. One thing that will help calm your nerves is simply to take lots of timed practice tests. Get enough sleep the week before your exam; sleep improves memory. Lastly, don't stress over any one question. No single question will make the difference between a 13 and 14, or 14 and 15. Be confident in your preparation, and visualize a high score.