Reviewing information (Q&A flashcards) over growing intervals. Eventually you would get used to this and can do it even on days you are bored of past papers and such. Taking advantage of the desirible difficulty effect: where stability of the knowledge goes higher when you are just of the verge of forgetting (which produces the best recall, as long as you actually recall it correctly!)
Without it you have no schedule to know, e.g. when you will study, when you will forget- you cannot know what you don't know, to the point that an algorithm with you only feeding whether you know or not know something at a specific time will out-perform your brain without it
Physical Leitner boxes are too messy and completely unportable. Use something like Anki or SuperMemo; outsource the inconveniences with automation (which is what top businesses do too!)
You create cards, then review them. If you get them wrong, you review the card a very short time later. If you get it right, the interval grows. (Default interval multiplier is 2.5 for SuperMemo 2, 2 for Leitner, and its more variable for newer algorithms). This comes from the spacing effect - basically if you DO manage to recall a memory on the verge of death, you actually make it much more stable, then if you for instance reviewed it each day.
Cram and dump may work for short-term tests - spaced repetition is more of a long-term investment.
If your school has mid-terms or finals then there often is a lot of material; this is its time to shine - probably make some material for yourself after each lesson or maybe at home (with a nap right after it to recover cognitive ability)