You're not going to be fluent in Japanese overnight. That's pretty obvious. But, there are a lot of little things that you can change pretty quickly that will get you one step closer to that goal of Japanese fluency. Alone, they may not seem like a lot, but when you combine them together over the course of several months, your Japanese will have become a lot better.
I've made a list of little things that I was able to come up with to hopefully help you out. Now, for the most part, these are little things. If you want big things, be sure to check out our Learn Japanese page which outlines better ways to study Japanese step-by-step. These are things that will save you a butt-load of time in the long run.
Japanese "R" Sound:
A lot of people have trouble with this. A lot of people also get lazy and skip over it after they don't get it the first time. The longer you skip over this, the harder it will become to correct. Trust me, you want to spend the extra time (an hour? Maybe two?) trying to wrap your tongue around this one so that your R's sound good later on. It's one of the biggest pronunciation issues Japanese learners have, and it'd be nice if this wasn't you, right?
Estimated Time: 2-4 Hours.
Effect: Sound more fluent / native
Get Better: "How To Pronounce The Japanese 'R' Sound"
Foreign Loan Words:
Loan words are foreign words that get converted to Japanese. They sound kind of like the original words, but since the Japanese sounds are somewhat limited in terms of what they can do, they can be kind of weird. The thing is, you know a lot of these loan words. For example, sutaabakkusu スターバックス is the word for "Starbucks." But, it doesn't sound like Starbucks. When you use a loan word or learn a loan word, don't get lazy on the pronunciation. It's too easy to fall back on the original "Starbucks" pronunciation for you, but it's actually really hard for a Japanese person to understand, meaning your Japanese is plain wrong even though your English is plain right.
Hiragana ne ね, re れ, wa わ, nu ぬ, me め, ru る, ro ろ, o お, and a あ:
These hiragana kind of look like each other. Beginner's of Japanese stumble on these over and over again, which slows them down from getting better at other things. Most of the time, these nine kana will be the weak point of people who have just recently learned hiragana. Less commonly, people will even have trouble with these for months (like, 6-12 months). Really though, all you have to do is sit down and destroy these eight kana in one sitting. Force yourself to recall them. Force yourself to write them. Force yourself to tell them apart. Make flashcards for them. Do what you need to do to make these kana easier than all the other kana. Otherwise, you're just letting them slow you down.
Estimated Time: 2-3 Hours
Effect: Fewer snags when learning hiragana
Get Better: Tofugu's Ultimate Learn Hiragana Guide
Learn The Rules Of Kanji Stroke Order
Stroke order isn't something you should learn for each individual kanji (if you even need to hand write kanji in the first place). Instead, you should learn the general rules of kanji stroke order, then apply them to kanji you're learning. This way you don't need to learn each stroke for each kanji. You just learn the kanji and automatically know how to write them correctly 99% of the time.
Estimated Time: 2 hours
Effect: A better ability to focus on kanji learning, the ability to write almost any kanji
Get Better: How To Guess A Kanji's Stroke Order
Learn How To Type In Japanese
Since people don't really handwrite Japanese anymore, it's a pretty good idea to learn how to type. First you'll have to set up a Japanese IME on your computer. After that, you have to learn how to type as well. There's a lot of little tricks and subtleties, but to learn how to type the main stuff is actually quite easy. With a little bit of practice, you'll be typing Japanese (including kanji) in no time!
Estimated Time: 1-2 hours
Effect: The ability to type in Japanese, which has countless side benefits
Get Better: How To Type In Japanese
Dropping The "U"
A lot of sentence-enders in Japanese end with an "u-sound." The most common are verbs (in both masu ます and dictionary form) as well as desu です. Although they end with an u-sound, most of the time you actually want to drop the "u" in these situations. When you get better at Japanese, you'll find some times when you can keep it on, but for now learning to get rid of it will make your Japanese sound better overall. It's one little thing you can change that will make a pretty big impact on how you sound!
Estimated Time: 1 minute
Effect: Sound better
Get Better: n/a
Solidify Your Katakana
Most people learn hiragana pretty well because they use it a lot. Katakana then comes along and is treated like the unwanted step-child. You feel like you have to learn a whole other "alphabet," not to mention one that isn't used as much as hiragana or kanji. If this is you, and you're at the point where you pretty much know katakana but it's not natural, spend a few hours (or 10 minutes a day for a couple weeks) making your katakana better. You want to be able to read it just as easily as you read hiragana. Kanji's already troublesome enough, and these 2-3 hours will make everything else easier going forward. It's kind of like those "look-alike" hiragana kana I talked about earlier, except an entire "alphabet" worth.
Estimated Time: 2-3 hours
Effect: Ability to read katakana
Get Better: Tofugu's Ultimate Learn Katakana Guide
Learn How The Common Particles Work
Particles show up all the time. They're in between everything. So, you should know how they work, at least in general terms. I'd recommend knowing and understanding the particles wa は, ga が, no の, mo も, he へ, wo を, de で, ni に, ka か, to と, ya や, yo よ, and ne ね. If you already have a moderate understanding of them, then that's good. Make it better. Particles should be things that you don't have to think about.
Estimated Time: 2 hours
Effect: Reading comprehension +1+1
Get Better: Japanese Particles Cheatsheet
This is just one of many one-time things that you can do to make your Japanese better. They all take less than 4 hours, so there shouldn't really be any excuse. Of course, this also depends on your overall level of Japanese. Some of these things will be fixed nearly instantaneously if you're at a higher level while others will take a bit longer if you don't have the prior knowledge necessary in the first place (if that's the case, be sure to integrate these tips into your learning as you go!).
So do you have any tips to add (I bet you do!). Remember, I'm looking for one-time things that will make your Japanese learning experience better overall, not things you do every day to get better (like listening to the radio, studying your SRS, and so on). Let me know what you'd recommend in the comments below! I'm sure your fellow Japanese learners would love to know!