Biggest thing you can do to prepare is prepare yourself for the likely chance that not only will you most likely not be a top student in your class, but that you can easily be in the bottom half of the class. Med school isn't like undergrad, you can't just study a bit and be top of your class with relative ease. Everyone here was top of their classes, people are going to be smarter than you in many different ways, people are going to have more background experience than you, just being able to be an average med student is a good thing - it means you are making it through med school.
Focus on changing your ego from "imtheman" to something more realistic, you may have been an allstar in college, but that is quite likely to change. When you get to school, just focus on adjusting to med school and succeeding in classes, if you are able to get all of that down fine, then start looking at ways to applying yourself to other things.