I know this is probably arrogant, but I wanted to write a few tidbits that I learned over the last two cycles of trying to do this. While I am 2/2 on getting to the OA I am still pretty certain that no one really has the inside scoop, even the great ones on the Yahoo board. There are too many people who speak with absolute authority on the process, and when you see people who are on the register either fail the written test or not make it past the PNQ you realize that no one really knows, and it can be a real crap shoot.
So with that as a backdrop this is my advice. Make sure that you understand the question being asked and answer the question. Really understand the dimensions being asked and make sure you address them in an engaging way that answers the question.
I have no way to back this up, but I also think the style of writing that leads to a passing score on the essay portion of the written test isn't necessarily the style of writing that leads to a good personal narrative. I know when I first sat down to write my narratives last year I tried to write a classic four or five paragraph essays and what I got was a lot of fluff, and couldn't adequately answer the question and show my personality in 1300 characters. Again I have to thank someone else for that bit of advice, but basically I shredded what I had written and essentially, for lack of a better word, just threw up on the page. You just don't have enough space to write the set-up or thesis paragraph that you find in a typical essay. Just get right down to the nitty gritty. That's what I have done both times. Remember it is a personal narrative and not an essay writing contest.
I think the other trap, especially if you are an older candidate like me, is to fall into the trap that the gravity of the experience or the personal resume will carry the day. I think that is wrong. I know that I looked for the "ah ha" sort of stories initially, but I don't think you need to have cured cancer or saved a comrade in combat to get the attention of the examiners. Remember that you are applying for a job as a generalist, and that technically you don't even need a college degree to become an FSO. So focus on the most engaging story that answers the question and shows the person who is wading through thousands of stories what makes you interesting.
I am a management cone candidate and where it made sense I tried to tailor the narratives to play upon my strengths as a management cone person, but I didn't try to force a square peg into a round hole. I think in both cycles two of my stories were more cone specific.
The other piece of great advice that I got was as you are writing think about how you would relate the story to a friend. I think this gets back to what works on the written test versus what gets you noticed during this part of the process. Again I have nothing to back this up, but some of the better, successful stories that I have read were more trivial in the grand scheme of life, but were written in a more familiar tone without being overly colloquial. The one common theme was the writer's personality came through loud and clear. I know as a former military guy we tended to write in an overlay formal and stiff way. Don't do that for sure.
Not that a ton of people read this stuff, but I hope that someone finds this useful. I know that I have gotten some great advice during this crazy process, and wanted to share what some really gracious people have share with me along the way.