My #1 piece of advice: LEARN the material, don't just memorize. Medical school will throw you a ton of information. Take a little extra time to integrate this information, tie major concepts together, and develop an understanding of how and for what purpose the body works the way it does. This will give you a strong foundation of a significant amount of the material that you will simply know. Then fill in the gaps with the picky details by memorization. Believe me, your grades will be higher, you will retain stuff better, and it makes the task of learning all that material much less daunting and even enjoyable.
Another piece of advice is to learn early on how to be a good test-taker. Good test-taking strategy can easily raise your grades 5%. Read the test questions carefully. You may have to identify the one correct answer and the one wrong answer on back-to-back questions, and some questions are simply just worded tricky. Medical school is all about the "best answer" on the multiple choice, so even if A) or B) sounds correct, make sure to read through the rest of the choices to make sure there isn't a better answer. It also helps to get a decent night's sleep before test day.
And finally, take everything your classmates say with a grain of salt. Many times I have overheard someone give an answer that they "know" is right when it really isn't. And although I get along with most of my classmates, there are a surprising number of people who are very good at being discreetly manipulative, people who are very strong-minded and have no common sense as to when to speak their mind, and people with over-inflated egos.
