For males, I'd say you should dress the same way as you would on any other rotation. On the inpatient unit, most people wear a white coat... if your institution is different, you'll notice when you get there and you can just take it off. Some psychiatrists dress more casually than other doctors, but you don't have the flexibility to make that decision as a med student unless you get there and notice that you're the only one wearing a tie. Some med schools might tell you to avoid wearing a tie or a stethoscope in the unlikely event that an aggressive patient uses it as a weapon, but if that's the case, you can always take it off when you get there.
For females, I might suggest dressing more conservatively than you otherwise would, since you might encounter patients who are hypersexual and/or disinhibited.
