Here is a hint: Be yourself.
If it is in you to research, volunteer, start charities, work in third world countries, etc..., do it. If you are doing it for the sake of making yourself look better, do not.
Case in point, I have been accepted to medical school. I volunteered at a children's hospital and worked in a hospital. I did so because I enjoy doing it. I just moved to Houston to do research over the summer. I also have put in an application at several of the hospitals at the Texas Medical Center to volunteer. My sister and I are going to start a charity to benefit children with autism in our home town. Why? I have already been accepted and no longer need to prove myself to anyone. I am doing it because I want to.
Sometimes I am saddened by the fact that people do things for reasons other than their own happiness. They say "the ends justify the means." If your end goal is medicine but you do not enjoy the steps along the way and only do them because it makes you look better then "the ends" should not be the ends for you.
I am not implying that anyone on this post, particularly wingy, who started this thread, is doing this for the wrong reason. I am merely trying to stress the fact that you should not do anything out of character to gain an upper hand in the admissions process. It is you who will be the physician and it is you, based on the true merits of yourself, who should appear on the application.