Hmm.... a few things.
1. Volunteer/ECs:
If I were you, I'd look very hard for two good quality ECs (one medical, one non-medical) to participate in for the next 3+ years. And make sure you really like both of them 'cause you'll want to spend a lot of time in each for the next 3+ years. Med schools seem to appreciate a few good experiences much more than a multitude of insignificant experiences. I also think it may be wise to have one medical for some exposure to medicine (duh) and one non-medical just to keep you a bit more rounded and to be able to say that you've had some non-medical experience and you prefer medicine over XYZ.
2. MCAT:
learn your science material well. It might also be a good idea to review an MCAT study book at the end of each class just to make sure your class covered what you need to know. For example, embryology seemed to be on the MCAT yet I never had that in my cell bio or physiology class so I had to study that on my own. In contrast, my immunology class was waaaaay more in-dept that I needed for the MCAT. Knowing what you need to know for the MCAT might be nice as your studying for your classes....if nothing else it'll motivate you to study hard.
3. Grades
get good grades. I can't stress this enough. Make sure you take classes with the "good" profs, schedule yourself wisely, and keep in mind the fact that you'll need letters of recommendation. So when you have that great anthropology (or whatever) prof sophomore year, make sure to mention your medical interests and keep in touch so getting a good LOR from him/her will be easy.
4. Research
do research if you want to. I went to a small undergrad intitution near a large public university where I did most of my research - this made it a bit more difficult for my advisor to write a LOR for me since she had never really supervised me in the lab. So I would suggest doing research with your advisor or at least in your dept. at your college if you can. Research doesn't seem to be critical to the admissions game, but it can help.
5. Have a life
Perhaps this is the most important point of all...