I personally use BunPro. It’s a grammar SRS site started by some level 60 WK people because they know it can be hard to learn and properly retain grammar.
They give a brief overview of grammar topics, and link to outside sources for more in-depth explanations. You then review grammar points by filling out the blanks in example sentences.
Otherwise: exposure, exposure, exposure. There’s only so many times you can look something up while listening or reading before it will inevitably stick.
You’ll remember it once you look it up enough times, just like in WK.
The other day, while reading, I came across grammar I’ve never seen before, ever. I did a web search for it, and more than half the search results were already-visited links.
I…had already looked up and learned the grammar (and likely recently at that). And then I had forgotten it.
Sometimes I’ll take notes on the grammar I learn, and note down which comic book, which volume, on which page I saw with. I’ll include the line of dialogue it was in, with a translation, and note down what that grammar means in the sentence. Then when I encounter it again when reading something else, I can look back at my notes, and seeing it used in a previously-read sentence/scene, I remember it better.
Recently I’ve been trying something new. Before reading I will study up on a grammar, I will then write it, and the previous 2 points I studied down on a piece of paper. Then while reading I pay special attention to find those piece of grammar. I’ve thus far found it a very effective way to retain grammar points
It happens to me all the time, but because it’s usually not a common grammar point so I forget.
Just adding my agreement that more and more exposure helps. I can’t tell you how many times I asked the poor members of the Beginners’ Club what って was doing in a particular sentence (it was a quotation marker, it’s always a quotation marker, why could I never remember that). But now I recognise it instantly!
The better you get at reading, the faster and the more you can read, and so the more frequently you are exposed to any particular grammar point. It’s a beautiful cascade of grammar exposure.
I just want to thank you for sharing this with me rather than just saying, “You gotta look it up, bro.” I’ve never heard of Bunpro before and it looks awesome and could be an actual study solution. For me, and I imagine most of us here, the problem with reading larger pieces is that it can get boring or tedious because it take like 15 minutes to finish a page because I need to constantly look up ever other phrase. Some times I just take the elementary school route and skip over stuff and try to use context clues (which is what you’re supposed to do) but there’s still the nagging idea that I want to understand most of what’s going on. Exposure through reading and then actually using the grammar that you see is really important, but using a service that uses proper grammar explanations (Japan Times Grammar Dictionaries; yes please) is really helpful. Thank you!
+1 for Bunpro. It made a colossal difference in my reading ability in just a few short months. I also use it as my first reference when looking up grammar points.
Another thing, besides exposure and Bunpro, is actually applying the newly learned grammar. If you’re speaking or writing Japanese on a regular basis, it helps to find ways to utilize that grammar in those situations. Sometimes it can be difficult or impossible to do this. In such cases, try writing or saying sample sentences on your own.
Even if you do this, you still may forget the points anyway. This may be due to the development of acquiring the grammar and normal for it not to come effortlessly.
I found that my Japanese really exploded when I started applying it in real life and getting real feedback from other speakers.
I’m quite new to bunpro but can already feel it helping. My main problem now is that with Wanikani, Kaniwani and Bunpro srs running I feel like I’m stretched to the limits of my learning. I am retaining what I’m studying but with a full time job, a 9 month old child and this it’s really challenging.
As many people here are saying, BunPro is amazing for learning grammar. It has a trial of 3 months (sadly i finished them already, and i still gotta pay) but i would say it’s worth it, if you have the money.
I don’t know if you are, but if you ARE going to drop one, I’d definitely say kaniwani.
It’s better to think in japanese than to translate everything from english
I usually take some of the example sentences found on whatever site you use to look up grammar and plug them into Anki. That way I’m seeing how the grammar actually functions in a sentence instead of memorizing the rules in isolation. And the SRS helps me to actually remember it long term.
I hope it will be as useful for you as it is for me! 
It is actually a 1 month free trial before you have to purchase a subscription, as far as I know. ^^
Except that sometimes it’s a topic marker! 
Bunpro is a luxury many Japanese learners of yesteryear would have loved to have.
Wait a few more weeks and scoop up that lifetime discount (if you like the trial). It’s a game changer for many as you can see
When is って a topic marker?
これってなに？？
Etc. It can function in some cases as a pretty casual は.
Ah short for というのは desu ne
Probably started like that, and then took on a life of its own.