So, I've never studied for the JLPT. I took N2 and failed, a year later took N2 and passed, a year later took N1 and failed and now took N1 again and don't know. I just study what I need to better understand manga, anime, and ラノベ (there is btw an app on the iphone by that title now that has a -ton- of material to read for free. Quality varies, but much of it is good enough that I would buy an actual book by the same author if I find one.) Ahem.
So the JLPT is just to benchmark myself, but if you really want to -pass- the test (at the N2 and N1 level) you need to make a point of covering two thing - reading speed and quick comprehension.
For reading, you have to read a -lot-. And you shouldn't read* manga (almost entirely dialogue!) or even ラノベ ... everything in the JLPT is non-fiction, mostly essays with the odd set of instructions thrown in (business memo, city notification, etc.). I know hundreds of adjectives and dozens of descriptive four-character sayings that are never going to be on the JLPT, but struggle a little with these kinds of things. (That said, the afterword in a fiction book is -exactly- the kind of thing that is on the JLPT.)
*(By "shouldn't read" I mean "shouldn't read as a means to study for the JLPT." By all means read them for leisure!)
For listening comprehension, you need to be able to understand -perfectly- moderately paced, standard (NHK and Tokyo classroom) Japanese. I'm glad I can understand what that Yakuza's guttural threat is actually saying and at least make out most of what the token Kansai or Okinawa character is saying... but again, this isn't going to appear on the JLPT. Erin's Challenge, NHK news, NHK 高校講座 are all available for free and provide plenty of practice with the right kind of material (although Erin's Challenge will -not- take you to N2, the simultaneous subtitles are really useful for bringing listening comprehension up to par if you have a tolerance for rewatching cute but slightly banal skits until you understand every word.)
Anyway, I got so focused on reading speed (and I completed 90% of the written section questions this time and have a high confidence in my answers so I should have passed that! Last year I might have answered 70% of the questions or less, but managed to squeak by in listening comprehension). However, I was shocked at how easily one or two missed words could leave me doubting the correct answer in the audio section .... especially if I understood the dialogue perfectly but wasn't sure what the verbally given answer choices were, or worst of all in one case I didn't understand the question!
Not sure if I just spaced out a little and missed a word, misheard a word, or simply didn't know a word but ultimately it doesn't matter. In English I can listen to an entire sentence, not process it consciously at all, and yet when prompted to answer 'replay' it in my head and figure out what was just said. I recommend aiming for that in Japanese if you don't want to sweat during the oral section.
Finally, the number one thing that I'm sure would have made the test easier that I didn't do ... practice tests!
I had no idea that my reading level was good enough to make the written section almost easy but that my listening was so poor still. I assumed that my anime and dorama watching would have kept up my listening, but there simply isn't enough of the right kind of material in fiction! (I perhaps should have spent more time with NHK news and 高校講座).
Even if the dialogues were all vocabulary I know (which they most likely were, the level in general didn't seem that difficult), it doesn't matter if I'm only familiar with them in writing. There's simply no time to think for a second or two and connect the spoken word to a written word you know because there's no pause, rewind, or repeat. You don't need to be able to understand at all fast speaking, difficult accents, dialects, or obscure words - but you need to -perfectly- understand average speed, average conversations from business and academia. That does include informal student-student and coworker daily conversations, although the majority are boss/worker, teacher/student, club-president/club-member conversations were you have to be sure of the assignment given. Also, the -order- of the actions that need to be taken if the assignment has multiple steps. 「まずは」 prefixes a lot of the questions where I'm not sure I got the right answer. I'm sure a time relation was expressed in the dialogue somewhere, but it wasn't always as obvious as 「その前に」at the beginning of a sentence.
Anyway, the point is that you won't come into the test unaware of your own ability if you take practice tests (written and audio!) a few times a year. It might not have changed anything for me anyway since I was really enjoying reading much more than listening to podcasts or watching TV and the JLPT is secondary to enjoying my use of the language... but if you're prioritizing passing, practice tests will help keep you on track for balanced study.
Edited: 2014-12-10, 12:37 am