Regardless of your knowledge of science and powers of deductive reasoning, if your writing skills are not up to par it will hurt your overall MCAT score and your chances of being accepted to medical school. On the MCAT, you are expected to show your capabilities of critical thinking as you argue in favor or against a given statement or essay topic. Practice shaping an argument in a sophisticated manner and providing evidence from a range of sources, such as literature, history and current events. A writing course with specific feedback from a professor on your strengths and weaknesses helps sharpen this skill.
A job in the medical or health sciences often revolves around mathematics. You need more than just a clear understanding of calculus, trigonometry and algebra; you need a comprehensive foundation in these concepts. Calculus, trigonometry and algebra review courses are extremely useful, or take a review class for the math on the MCAT. A class like this shows you how theories in calculus, trigonometry and algebra manifest themselves on the MCAT and how to address those questions.
As much as the MCAT involves having a strong background in math, this is even more the case when it comes to chemistry. In college, take all necessary chemistry pre-medical courses. While these vary from university to university, the basics are more or less the same. Take fundamentals of chemistry, and inorganic, organic and bio-organic chemistry courses. Once you've taken those courses, take another that not only reviews all these themes but gives you examples of the questions on the MCAT.