I don't know exactly how they do it at our school-- I know the top 15% eventually get in, and they select a few from that group to be inducted in the spring (ie, before residency applications go out). Good grades can't hurt, and neither can getting involved in whatever interests you at school-- research, community service, etc.
On a related note though, for you aoa people, could you share what you wound up getting for an insignia/certificate? I don't know if I'd ever wear anything on my lab coat (no one does that here at least.) On the other hand, maybe I should pin it to SOMETHING so I don't lose it...