As the previous posters have said, knowing your fundamentals is key in the first place. However, doing well on the science passages means recognizing that you DON'T have to comprehend everything. Get a general picture of what's going on and understand key points, which are usually the ones that can be most easily apprehended through a combination of a few basic science factoids. If something seems too weird, don't let it hang you up. Move on and hit the questions--A lot of times, the questions will side step the strangeness. If not, the questions often clarify the passage by giving you a motif of the main idea (say, if the questions keep talking about what functionality is preserved and what is lost when nerve X is lost, then you have a general idea that the passage's key points include the idea that you can sever a nerve but keep certain functionality).
After lots and lots of practice (which is essential), you start to get a feel for what the AAMC thinks is important. That is, you start predicting the questions as you read. You won't be 100% on target, but you'll often have a general idea about the direction of the questions. I often found myself getting a little nervous while reading passages that a question would address a certain basic science concept touched on in the passage that had slipped my mind, like some silly thing I should have memorized; later, I would discover that that which worried me was not addressed.
You'll also begin to learn how to use question stems and answer choices to your advantage to better understand the passage. This will include knowing when answer choices are trying to trap you into a certain kind of thinking and being able to figure out when choices can be useful to you and accurate. For example, many times, all answer choices presuppose a certain condition that might not be trivial (at least to you). Then, if you were unsure about the status of that piece of information, the inherent assumption of the answer choices can guide you w/respect to the passage, provided that the assumption was not introduced in the question stem itself.
With lots of practice, what I'm saying and a lot more will become more clear. It does take that practice and a fair bit of knowledge, though. Once you have prepped well and tested your preparation and understanding of basic science through lots of passages and exams, you'll come to realize that the technique by which you best approach passages becomes second nature and that a lot of the strategies I have outlined here become a natural part of your thinking process.