If you're applying for a surgical residency—General Surgery, Urology, Otolaryngology-Head & Neck Surgery, Neurosurgery, integrated Vascular Surgery, integrated Cardiothoracic surgery, or Plastic Surgery—it's almost a given that you'll be doing a sub-internship, or a "sub-I." (and sometimes called an "AI" or an acting internship). That being said, many other specialties such as Radiation Oncology or Dermatology, encourage doing away rotations/sub-Is. Sub-Is are obtained through VSAS, or the Visiting Student Application Service, and are usually for a month. You apply for institutions participating in VSAS towards the end of your third year.
While there are many points to doing a sub-I, the whole point of the sub-I can be distilled into two things:
1.) Getting a recommendation letter
2.) Increasing your opportunity of matching at that certain institution.
There are always stories of students completing a month long sub-I only to find that they were not offered an interview at that institution. That being said, there are also many students that match where they rotated.
Know what you're getting yourself into
You are at your away rotation to make a good impression, both for yourself and for your home department. Expect to work long hours as a sub-I. Expect to get there before the intern and leave after the intern leaves. Expect to get the numbers and keep the list updated. Expect that you will probably not get the most sleep. Expect that you probably won't eat that well. Expect that you may be working weekends. Expect that you won't have time to check out the city or to meet up with friends. Expect that you'll be reviewing for cases or clinic the night before.
Prepare for clinic
If you can get access to the clinic list the day before any given clinic, take a look at each patient's chart. Skim through the post-ops or return visits and quickly jot down what needs to be done. Read through any previous notes on new patients that are coming in. Look up their pathology on UpToDate or Dynamed or glance through a review text. While every student will prepare for cases, preparing for clinic puts you in a different league.
Prepare for cases
That being said, ensure that you prepare for cases. This includes reading up on the patient's history, knowing their exam and imaging, reading up on relevant anatomy, and reading up on the procedure for the case.
Be ready for rounds
Stock your pockets and bring a bag, if necessary, with dressing supplies, suture and staple removal kits, and tape. Always keep a stethoscope and a light on you. If you're really fancy, you'll bring a headlight to help with dressing changes and bedside procedures.
Check on the patients
If you have time in between cases or clinic, pop your head in patient rooms and check on how their doing. If there were any studies or labs ordered during morning rounds, periodically check on them and see if they're back. This will not go unnoticed.
Ask for a recommendation
Ask for a recommendation from someone well known in the field, preferably the program director or chair. Be warned, often these individuals will have pre-made template letters and if you haven't worked that much with them, these cookie-cutter letters won't contribute anything that special to your application. Choose an attending you've worked closely with over a chair/PD letter that you've only been in clinic once or twice. A good time to ask is about half way through the rotation. If you want, you can give them a heads up that you'll be asking for a letter so they know to keep an eye on you and your performance. Don't worry too much about asking; everyone knows you're there to look good and to get a letter. Field out from the residents who are good attendings to ask and try to get in more of their cases/more of their clinic
The whole thing, distilled down, is basically this: Work hard, don't complain, and be yourself. Happy rotating!