*Proper-fitting shoes with adequate arch support
*Stand up straight (every med student slouches or bends over in order to get their face closer to the field)
*Stretch your legs/back from time to time
*Don't lock your knees
*Strengthen your abdominal muscles/core
All of the above and also one more thing.
Make sure that your glasses/contacts prescription (if you have one) is as accurate as possible. Having your eyesight be off will cause you to bend over a little more to see more clearly and worsen your back pain.
Well coming from a fellow medical student, I would definitely agree with all the attendings are saying. One thing I found useful was actually trying to stand up straighter, with good posture. It really helped with back pain, especially when you are wearing lead. Plus I got custom fit orthodics (sp?) in my shoes and they make a world of difference!