"If you could go back and do things differently as an intern..."
One:
I actually had this thought after the second month of my 3rd year in med school. What I felt, then, was that I needed to be really prepared before internship - meaning I wanted to know the job and what was expected of me before I signed the contract.
The most important thing: pick the right hospital, with the right attendings. Visit with, talk to, and interview previous residents about specifics of their experiences.
Two:
Get rid of everything in your life that you don't need for residency: possessions, deadweight boy/girl friends, bad habits, negative additude, etc.
Three:
Cash, have plenty of it. Work for it prior to med school (during college), save it, and budget yourself during residency. I had enough money for daily life during residency, but I also lived with four roommates, bought second-hand, and did not waste dollars on "stupid crap" (you get to determine stupid crap).
Four:
No relationships at the hospital. Period. Ever seen "ER", "Grey's Anatomy" or "Scrubs"? Keep your libido in check.
A bad residency, especially the intern year, hurts for years...