Some good advice above. Other things:
-You can never be early, only late. Show up when the intern does (show up before you're told to), leave when the intern does (or after you've been dismissed by multiple people).
-Try to be helpful and interested without being annoying. From the other side, we love a curious and interested med student, but not one who doesn't know when to shut up (e.g. it's an emergency and you're interfering with my work to ask me unrelated questions). Examples of being helpful would be getting numbers (vitals, labs, I/Os, whatever) on patients in the morning and having those ready (and correct) for the intern when they arrive or offering to hold the pager when you aren't scrubbing a case.
-If you're going into a case, know the patient, why they specifically are getting the procedure, and generally what will happen during the case.
-Practice suturing, look up how to close drains and remove staples, etc.
-Depends on the program culture, but try to pick up on any unspoken rules. Don't one-up anyone on rounds - if the junior gets the pimping question wrong, don't blurt out the answer. At my med school, senior surgery residents were called "Dr. X" or "sir/ma'am;" on OBGYN only the senior residents wore makeup/jewelry. (Some) attendings were only spoken to when they directly asked you a question. Etc. Like I said, it all depends on the culture.
-Finally, be tough. Might not happen on urology, but part of the surgical experience is someone being mean to you. Your responding well to negative criticism is huge. Just because you got yelled at doesn't mean you've blown it, it just means you have something to work on for tomorrow.