There are a ton of topics I learn/read on a daily basis. Some are out of my personal interest, some out of need (when interviewing).
I notice that after a while, without enough usage, these topics simply evaporate from my memory. I know the simple answer is to keep practicing.
I’m interested in knowing how different people do this. Looking for any tips, hacks, etc.
I don't know why nobody else has suggested this.
If you want to learn, really learn something. Self educate, then teach it to someone else. You'll have to really educate yourself on the subject, but once you do.. Blog post, Twitter, Youtube, Linked-In, or whatever platform you want to use. You'll get corrections if you're wrong about something, and hopefully a load of questions that will make you dig further into the topic. The process of teaching it will reinforce it into your memory as you present it in the way you understand, and the feedback you get will also strengthen your understanding even further. You'll become much less likely to forget it over time.
But what if you're like me, and you don't have a social media presence or following to get any kind of feedback? Surely you have friends with similar interests, or know one of those people who likes soaking up knowledge like a sponge. You could use HN here to post something, or post to Reddit's "Today I Learned".
Inevitably, teaching something is the best way to learn more about that thing, and learning more about it is what will make it stick better.