I didn't know how much people had prepared for the Foreign Service Officer exams until after I'd passed the oral assessment. There were five of us who passed, of whom I was the only woman, and one of the other candidates mentioned that he'd been part of an OA study group for the past few months. Wha?! Then it turns out that there are several Yahoo!Groups devoted to different stages of the Foreign Service recruitment process, in addition to "boot camps," study groups, Skype language tutoring, study guides, and so on.
My own personal preparation consisted of (1) reading through State.gov (2) reading through the official study guide and (3) going to two info sessions, one at Peace Corps career services, and one at my grad school. In other words, hardly at all. And yet I passed with a solid score. (I don't want to jinx my chances, but my score was above the minimum cut-off, plus I got extra for passing French.)
I guess I was super-laid-back, because I knew that I could always go overseas again in some other function, or accompanying MoBob. Also, I had numerous personal resources - my grad school (duh - grad program in international relations), ICAP, Peace Corps, church (Mormons are well-represented at State) - that not everyone has access to. I guess it's not so much a miracle that I passed, but more of a lucky convergence.
eta I did take (and pass) the Foreign Service written and oral exams before, in 1991, the year I graduated from college, and the written in 2000 in Morocco (when we weren't going to be in the U.S. for the orals). So long ago that I don't remember much, and so different (on paper!) that it was like starting over again with this round.