Stethoscope.
Fine point pen.
A good attitude.
Sometimes more, but never less.
Oh yeah, definitely have LOTS of pens on you. Enough to hand out to anyone who needs a pen at a moment's notice. One day on Trauma, I went from 3 pens in my front white coat pocket to 0 over the course of a day. Don't expect them back from your superiors, either. If they give it back to you, it's a bonus.
Also this is the basic rule of pimping:
If you get pimped and get it wrong (as long as it's not super obvious) that is relatively OK. Now one of three things can happen:
1) The superior tells you the right answer. This is the best scenario for the med student, as you now do not have to look it up. Although looking it up in better relation to your patient, etc. etc. may not be a bad idea.
2) The superior tells you to look it up. This means that you sure as hell better look it up sometime during the day or when you go home at night. They will 75-95% of the time not ask you the same question again, but if they do, and you don't know the answer (or at least AN answer, as some attendings want you to read their minds), then you'll look like a clown.
3) The superior does not tell you to look it up. This is a trap. Look it up just like in number 2.
If someone is doing a true 'pimping session', this is what will happen. They will generally start off with easier questions, then get into harder ones. They want you to screw up at some point so you have something to self-teach. Sometimes the first question they ask is a topic I was unfamiliar with, so I felt really dumb. In a group setting (say IM rounds) let whoever the question is directly asked to (even if it's an intern/resident) have a shot at answering it first. Of course, if there is no direct person that is asked, feel free to offer your thoughts.
Basically, don't be a dick to your team or make them look bad, and the residents/interns will hopefully do their best to make you look like an all-star. IM Pre-rounds (with the resident) were the biggest help to make me look like I somewhat had a clue of what the hell was going on before Attending rounds.