The Leitner System was designed by German scientist, Sebastian Leitner, and involves studying with flash cards and five boxes. All your flash cards start in Box 1. When you revise a card, if you get it right, the card moves to the next box (Box 2). If you get it wrong, it goes in Box 1. The next day you study the cards in Box 2 (the ones you answered incorrectly the day before). If you get it right, it moves to the next box (Box 3) but if you get it wrong it goes back in Box 1. 
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You revise the boxes on the following days:

• Box 1 — Every day
• Box 2 — Every 2 days
• Box 3 — Every 4 days
• Box 4 — Every 9 days
• Box 5 — Every 14 days.

According to Very Well Mind*, the right amount of stress can increase your memory performance. But too much stress inhibits learning because the stress hormones that are released impede the formation of memories. This is not helpful if you are trying to learn a new concept or recall an old concept for a test. Stress can also be exhausting which makes it difficult to learn. The good news is there are things you can do to reduce your stress. The Smiling Mind app has free guided meditations and mindful activities - some as short as three minutes that you can do before and during your study. 
* https://www.verywellmind.com/stress-and-your-memory-4158323

A balanced diet that is high in fruits, vegetables, legumes and whole grains and low in sugar and processed fats is important for good health. The following foods are particularly useful for brain health:

• green leafy vegetables such as spinach, kale, broccoli
• fatty fish such as salmon and tuna
• berries
• tea and coffee
• walnuts

Don't go on some random spinach only diet, balance is important!

When you cram, the information you review does not move from your short term memory to your long term memory. The short term memory (as the name suggests) only stores information for a short time. It has been likened to a sticky note. Sticky notes are temporary things - they are fickle and often end up stuck to your sleeve or fall to the floor. You want to move this information to your your long term memory which I imagine is more like a set of encyclopaedias or the hard drive on your computer. This happens when you review the material in chunks on different days, rather than cramming it all in on one day.

I know it's an obvious one, but I also know that lots of you aren't getting enough sleep. You would be better off doing less study and getting more sleep - that way the revision that you have done will be more likely to stick. If you cram in more study and stay up late, you are likely to forget what you revised. 

You learn something best when you expect to teach someone else. The best way to do this is to actually teach someone else. If you can't find any willing victims (I mean, volunteers) you could always try a stuffed animal. Perhaps you could get together with some friends and teach each other a different topic.