Not OP (and I don't have much time), but as of about 5 years ago (last time I was spending time reading papers), the biggest thing I ran across was the spacing effect (which is different than spaced repetition). You may have heard of spaced repetition. Very quickly whether or not you remember something is a random variable with a vaguely gamma distribution. Your chance of remembering is a function of how long it was since you last looked at it. So if you wait 5 minutes, you might have a 90% recall chance, whereas if you wait 5 days, you might have 20% recall chance. The speed that the recall chance goes down is a function of how many times you have reviewed something and how much time has gone on between reviews. Better than trying to understand my terrible description, look up "forgetting curve".

Anyway, in spaced repetition, you usually try to time your reviews so that you have a 90% recall chance. This minimises the amount of time you have to do review. Also, initial remembering of something takes a lot of time, while reviewing something you already remember takes very little time, so you try to make sure that you review it before you have forgotten it. Usually at around 90% recall rate, after 12 reviews, you're many, many months between reviews, so it's insanely efficient.

However, there is a completely different thing called the "spacing effect". What that says is that if you forget something, and then relearn it, the forgetting curve will be much, much, much shallower than if you had done the spaced repetition (papers on spacing effect don't put it that way at all, so you're getting a big dose of my bias -- better to do some research on it. But I'll continue anyway since I think this is the easiest way to understand it).

What this means is that you can remember things much, much more easily if you forget them first and then relearn them. So in this school of thought (literally -- ha ha) you learn something, then you wait until you forget it, then you learn it again. In studies, this has been called the most profound learning effect every seen (pretty powerful words!)

So there are some other people who like the spacing effect thing and thought there must be a better way than randomly waiting until people forget stuff. So they came up with an idea called "interleaving". In interleaving, instead of learning one subject for and hour or two and concentrating on it until you understand it, you study something for 10 minutes, then study something else, and then something else, etc, etc, etc. After 6 or 7 goes of this, you circle back to the original thing and study it again. The idea is that by keeping distracting yourself, you have no way of remembering anything at all -- which speeds up the spacing effect. (Though, one might note that this starts to get into the realm of spaced repetition -- and I think it's not a coincidence).

Basically, in interleaving you try to mix everything up and make sure that you are constantly looking at something new. But, of course, this sucks. I mean it sucks enormously. You never feel comfortable. You can't remember anything. As soon as you think you are getting the hang of something, the topic changes and you freaking forget it all. It's hell.

Which has led this kind of thing to be referred to as "desirable difficulty". If the studies are to be believed, the potential is incredible. I'm not sure I believe the studies completely, but I've experimented on my own students (when I was teaching) and I was pretty impressed with the results I was getting. Still, really early days with this research, though and I wouldn't be surprised if someone replies to this message with something like, "Yeah, somebody found out in the last few years that it's a load of BS". Education and psychology are pretty damn hard to do studies on ;-)