I shadowed a couple doctors in different specialties when I was doing my premed work. They were interesting experiences, and helped me to see what it might be like to work in those particular areas.
The main thing you'll probably do is observe the physician during patient encounters while he/she takes histories, performs examinations, or reviews the results of studies. If there's some down time, he/she might discuss an interesting case or clarify something that happened during a patient encounter that you observed.
Since your role is likely to be
very passive, there's probabaly nothing you need to do ahead of time to make sure it's a positive experience. However, if you want to go the extra mile, here are a couple things you can do.
1. Read something about what it's like to be a doctor in that particular field. Then you can compare what you've read to what you observe in the doctor's office. You can find online material, such as
http://www.aamc.org/students/cim/ or
http://www.acponline.org/srf/res_progs.htm?hp (which has a lot of information on careers in Internal Medicine).
2. Try to learn something new each day you are at the doctor's office. You can do this by identifying one interesting question that you want to learn a little more about. Do some additional reading to on that issue after work (maybe 15 to 30 minutes). The next time you are at work, talk about what you learned with the doctor. You can easily use an online resource like
www.eMedicine.com, or some appropriate medical book.