1) you are there to learn, but always ask questions after you leave the patients room
2) use hand sanitizer before and after you go into the room, if they have it
3) when you are actually observing, watch from a distance unless the doctor tells you other wise. Don't speak unless spoken to.
4) don't try to be witty or funny or knowledgeable if the patient talks to you. Be courteous.
5) for the love of god, don't introduce yourself as doctor to patients (heard of this happening before)
6) don't ask for/wear a doctors coat. Khakis and a nice polo are fine