As I said before, doing well in every class is essential. There are so many good students out there competing for so few openings, so you need to have the best grades possible. Otherwise, you won't look good enough on paper to make it to an interview, where you can impress them in person. Also, try to get involved in as many extracurricular activities as possible without compromising your grades. They should focus on volunteer work, health-related programs, or community or ministerial service.
Research is also very important, and is looked upon very highly by admission committees.
They don't want students who just study, or just volunteer, though. Medical schools want well-rounded students, so it's important to stay active in your hobbies, whether intramurals, hiking, playing video games, etc. As an example of this, I even had one interviewer ask if I preferred the Sega Dreamcast to the Sony Playstation, and then continued with a conversation about the best games for each system. So anything can happen. Of course, the MCAT is important, and preparing for this test should take at least six months of studying.