Black shirt, black tie, black pants. But here's the kicker...offset it with a fuscia headband. They'll melt for you.
Seriously, though:
Shirt, tie, white coat the first day. If on the first day you observe that none of the other students or residents on service wears a tie or a white coat...you still wear a tie and a white coat every day to prove you're serious.
People actually notice that kind of thing, particularly the older people who (1) care about appearances and (2) decide your fate.