Slacks, Pants and a Tie (man).............honestly its a first impression,you can always dress down if needed, I was wondering the same thing when I was shadowing, I wore nice slacks, button shirt and a tie, I thought I would have been out of place, until I went to the office and found that everyone was wearing the same atire.......I is after all a first impression and some one is inviting you into their domain to watch them so it is good to look nice