first, talk to other students who've done a surgical sub-i at the same hospital. see what the expectations are: at some places you'll work as hard as an intern, but at others you may have fewer real responsibilities, other than perhaps more time in the OR.
get a good surgical handbook (mont reid, etc). you should know recall by memory by the time you start the rotation, since it really only covers 3rd-year-level stuff. get a good surgical atlas, and read up on the most common cases. if you're on a more specialized service (eg hepatobiliary, vascular, etc), be sure to know everything about those cases. if an attending has a special interest, or is an expert in a certain type of case, be sure you know everything about those cases too.
know how to tie knots, suture, put in IVs, etc. (all the skills you should have learned as a 3rd year. -- many students at "cush" hospitals get little real practice in these skills, so be sure to brush up on everything if you're a little rusty)
be sure you get organized and have a system that works for you (checklists, etc) to cover all patients on your service when you're on call. know all phone numbers since you'll need to call other services and follow-up on studies.
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