All the President's Men
The title is the name of a good movie that shows how investigative reporting actually goes about, using the Watergate Scandal. Another good one is the focused investigative reporting in "American Gangster".
This is one of the most important facts for me, not because I break the law all the time, but because I don't know law very well or where the boundaries may lie. I know that impersonating a police officer is wrong, but what about someone else? What about a professor? Or like Nellie Bly, a crazy person? Are these illegal? Or is it just illegal if you give someone else's name? Or is that also legal?
You see my dilemma. And that is just in one part of digging up dirt.
Another thing I didn't know that is "Work within the law"-ish is not to conceal criminals or pay for tips. You always see this in cartoons or movies: a journalist/reporter/private eye pays off another crook (who is sometimes on probation) for information about greater crimes. He let's this criminal go because he'll be able to keep going back to him for street information (at least until the criminal is killed for being a snitch).
Another idea with investigative reporting is avoiding personal comment. I would extend this to even quoting people that are just commenting on the situation rather than an authoritative voice because then you may seem biased if you just get negative responses. At the same time, if you are unfolding a scandal the opposition is less likely to even speak with you let alone give a postive comment on the situation.