To get better at Japanese as a whole, you need to practice every aspect of the language on a regular basis. That means reading, writing, listening, speaking, AND knowing under what situations/conditions certain things are appropriate and inappropriate (this often gets left out of study guides). No point in systematically learniing keigo if you aren't also taught that it can actually be inappropriate to use at certain times, for instance. For example, overusing keigo with people that are on your "inside" circle of people (family, friends) can actually come across as patronizing or insulting. Anyway, I'll go ahead and tell you what I do, and you can maybe see if it works for you.
First thing to know is that Japan loooooves qualification tests. There are qualification tests for pretty much every aspect of life, professional and personal. I totally would not be surprised if there is a test for measuring how well you can wash dishes or do laundry. I personally really like this system, and I focus my overall goals around qualification tests. I don't believe, however, that the JLPT is an accurate measure of skill, which is why I think it's also a mistake to solely study just for one qualification test. I just use qualification tests as an overall goal marker. One thing to know as well is that you need at least JLPT Level 2 before any serious Japanese company with a Japanese environment will consider you, if your goal is employment in Japan that isn't English teaching.
There are three Japanese language tests that are very relative and very important:
- Japanese Language Proficiency Test (JLPT), which you already mentioned
- Kanji Proficiency Test
- Business Japanese Proficiency Test (BJT)
What you choose as a goal marker should depend on what your overall goals are. If you just want to get a job in Japan, you can safely focus on just JLPT and BJT, skipping the kanji test all together (that doesn't mean you can skip studying kanji, though!). If you want to study the language as a whole for personal enrichment/knowledge, you can focus on the JLPT and the kanji test, skipping the BJT all together (that doesn't mean you can skip keigo, though!).
My personal method of study is pretty lifeless, but it works for me. Basically, I just pick one qualification test (say, Kanji Level 4, or JLPT Level 1), I pick up a book or two on JUST that topic, and work through it, cover to cover, no excuses, no burning out.
Then to supplement that as I'm going through it, I read one Japanese news article per day. But as I read it, I also copy it by hand into a notebook, even the stuff that I understand. This is particularly helpful in just keeping writing skill up, since you rarely need to actually write kanji by hand anymore. I also go through the article, and without any self-deception or cutting myself any slack, ask myself, "do I understand this sentence 100%, including all grammar, vocabulary, and kanji?" The answer is quite often, "no." Even if I "get the idea" or understand it 99%, there's most often at least some kanji that I know the on-yomi for, but not the kun-yomi, for example. So I ruthlessly then make flash cards for every single element that I didn't know, whether it be a grammar point, or just one kanji.
For me, flash cards are key. Primarily because by MAKING the cards myself, I end up researching and writing out the content I didn't know, and by just doing that, I pretty much memorize it before I even have to actually use the flash card.
Listening and speaking with Japanese people is the best way to get your listening and speaking practice in. The key that most people screw up, is they don't stop a conversation when the other person says something they don't understand. Nodding your head and smiling is a huge mistake. If the other person says something you don't get, don't make an assumption that "it probably means this or that," stop the person right there and say, "sorry, what does that word mean?" I've never met any Japanese person who isn't happy to help out.
However, if there's not always a Japanese person around to practice with, Japanese TV is your best friend. Not only is it super entertaining because it's so whacky, most TV shows are like an hour or less, so you can watch the same thing several times to fill in the blanks without burning out. The key is to watch the same thing more than once, and to imitate what you hear, keeping the inflections and tones intact. Works best if you're alone, so you don't get embarrassed.
So I do the above, consistently, every day, mixing things up until I finish the book for the test that I'm aiming to pass. By that point, I can pass the test with flying colors, and I learned a whole bunch of other stuff along the way including new speech patterns and new vocabulary.
Like I said, this all totally depends on personal preference and how you learn. The key to any method of learning though I think, that I see time and time again, and that I'm sometimes guilty of as well, is self-deception. Don't study for the sake of getting to the end of a chapter so you can stop studying. You have to really want to be studying. You also can't use excuses to not study, like "oh I have a cold today, I'm just going to take it easy today," because then you start doing stuff like that, and eventually you find excuses to work yourself down to like one or two days a week, and then eventually you just stop, saying you'll come back to it soon, but never do. The key is consistency, not letting up, and being hard on yourself.