You should probably...
1. Shadow dentists for at least 50 hrs, 100 preferred but the more the better.
2. Do research if possible. Try to contribute as much as possible rather than say, dish washing. It would be even nicer if you have publications.
3. Volunteer at hospital/clinic.
I'd say shadowing is a must have, but if you have all of the above then you shouldn't worry much about your EC.