It is important that your experiences and activities be an ongoing process. Do not start your extracurricular experiences right before applying to medical school. This will be viewed as simply getting the experiences because you have to, instead of doing them because of your own interest or desire.
Also, keep in mind that the applicant pool is filled with thousands of other highly qualified applicants, many of whom have known from an early stage that they wanted to become doctors, and who have a considerable amount of relevant activities. These are the people with whom you are going to be compared and against whom you will be competing for a limited number of seats.
This post is excerpted from 101 Tips on Getting Into Medical School by Jennifer C. Welch, who has served as the Director of Admissions at SUNY Upstate Medical School since 2001.