We started with this sentence:
Before going on your informational interview, it is crucial to be prepared for it.
I see a couple of problems in this sentence. First, it's redundant: preparation always happens before something, so you needn't tell a reader to prepare before an interview—you can just tell her to prepare.
Second, I dislike the repeated pronoun it, because it stands in for two things: "interview" and "to be prepared for it." Sentences like this can often be shortened. "It is nice to eat cake" can be "Eating cake is nice"—which is shorter and seems more vivid (because the subject is at the beginning of the sentence).
These two problems conspire to make the sentence needlessly long. A few quick keystrokes returned this edited sentence:
Properly preparing for your interview is crucial.
I cut informational because the title of the article is "The Art of Informational Interviewing"—so the reader already knows what we're talking about. I added properly because I liked the sound of it (and it is possible to prepare poorly for something).
I think this sentence says what the writer wants to say, without a lot of unnecessary guff.
Among other small edits, I changed the recommended interview question "Who do you work with?" to "Who works with you?" (to avoid the whom morass altogether).
Read "The Art of Informational Interviewing."