The biggest mistake students make is spreading themselves too thin. They join every single pre-med club available, participate in a handful of sports, jump from one research project to the next, and travel around the world providing medical care to rural communities.
This tactic often results in students committing minimal amounts of time to each activity and gaining red flags instead of praise on their applications. Instead, be choosy with your extracurriculars and ensure you are committing to each activity for a solid amount of time.
Those who commit years to few endeavors will catch the eye of the admissions committee over those with a plethora of short-lived activities.