Your best bet is to read the textbook. NREMT is not a secret mystical test where only the lucky pass. Pretty much every paramedic textbook written is to prepare you for the NREMT (not that that's all that they are good for). Browse through the forums, numerous people have struggled through the exams and have reported tips here.
My personal recommendation: pharm, OB and peds. People normally struggle through these topics and they are often not covered in sufficient depth by a lot of paramedic programs. Most schools teach the drug formulary of the local agencies, but NREMT doesn't know or care where you will be working, you'll be expected to know every drug written in your paramedic textbook. I don't mean just their names either, while I've seen some programs teach "amiodarone is your second drug in cardiac arrest", NREMT will expect you know what class anti-arrythmic it is, dose, route, MOI, everything. Even for stuff you probably can't give, like labetalol and mannitol.
I think OB and peds just simply gets glossed over because many instructors don't know, or understand it themselves or, like me, are male and have a difficult time caring what the placenta is doing in the 2nd trimester. If it were up to me every patient that said "I'm pregnant" I would simply toss up my hands and find a nearby female. Alas, there are none to be found, barring the patient.
Oh and the best advice: relax. Many people defeat themselves before the exam even starts. You get, like, 4 tries before you need to take a refresher or something silly. 7 total attempts I think. Don't psych yourself out before you sit in front of the computer.
Also, low passing rates are rarely the faults of the students. Good schools have good passing rates. Period.