I would never profess to know how best to prepare for the journey of becoming a foreign service officer since I am 0 for 1, but as I am beginning to gear up for round two here are my thoughts. My scores when I took the test were about 59 and 59 on the JK and BIO section and a whopping 48 on the EE and an 8 on the essay.
My impression is that most people spend a disproportionate time studying for the JK section at the expense of EE and BIO prep. The JK is the sexiest part of the test and if you have taken it once you know that it really isn't. I know that during my first prep cycle I probably spent too much time on that part as well.
I think you will find that while you can study for the JK section you could study for a year and not see one question from your prep session. I think you need to take more of a long term view in terms of how you obtain the type of knowledge to do well on the JK section. I absolutely had a few questions on my test that I ran across from several of the books that I read so studying can have its benefits, but this part of the test doesn't lend itself to cramming.
The one section that you absolutely can prepare for is the EE section. Going into the test the first time I knew this was a weakness and while I did put in some time it wasn't enough. It is the standard take an awkward sentence and make it less awkward. I can't recommend a particular book, but if you look at the sample questions that ACT provides you are in the ball park and any study materials with similar sorts of exercises would be helpful.
The BIO section is the wildcard with the biggest delta and is a mystery to just about everyone who has taken the test. The one thing I think I can say with absolute certainty is that how one typically views "experience" does not translate into a high BIO score. If I had holes in my volunteer portfolio, community service, multi-cultural experiences, etc, I would really think about what could I do to fill out my personal portfolio? The other thing that I think is important to remember is that you didn't have to cure cancer or save your platoon in Afghanistan for the experience to qualify as being noteworthy. Don't forget this is a generalist, entry-level position. We are not applying to be the next ambassador to Russia! So don't be over confident if you have a lot of experience and don't sell yourself short if you are still wet behind the ears.
While I would never discount the fill in the blank portion of the BIO section, if I were a betting man I would bet that is much more important which bubble you check rather than what you write. So before the test and the months leading up to it write down the broad categories and the free practice questions give you a decent list, and tick off examples. If you think you may be deficient in an area do something about it.
I know someone that just failed the most recent test(did not pass the essay) and he passed the OA before, but at the time decided not to go forward with State. I have seen his writing and he helped me edit my successful personal narratives during the QEP phase so I know that he can write well. Even if you are a strong EE person do not take the essay portion of the exam lightly. I did wait to cram for this portion. During the two weeks leading up the test I wrote 1-2 timed essays every day to get my brain and fingers prepared for test day. I used many of the prompts from the ACT/State material, the Yahoo board, and picked topics from the NY Times and Washington Post. I also thought about several controversial topics and believe it or not that is what I got on test day. It wasn't exactly what I had thought about, but close enough for government work.