Because we like doing easy things—so our tendency, when we revise, is to concentrate on the things we already know. You could try using this to your advantage by dabbing your wrist with a particular perfume or aftershave each time you study material for a particular exam.
It sounds obvious, but it's surprisingly hard to do.
When you have written the essay make sure it is grammatically correct as you will be memorising every comma and full stop. I memorized dozens of essay plans this way when I was a student. Mentally connect an image of each of the features you see — such as a chair in your imagined house, or a tree by the side of your imagined road — with a manageable chunk of information such as a chemical formula.
Icy Uranus and Neptune follow, and, bringing up the rear, Humble Pluto — no longer a planet — sheds a tear. It makes sense to practice exams like you'd practice a sport or a musical instrument. Get organised Tidying your desk should be top of your to-do list.
Armed with this information, you will at least know what you need to know, even if you don't know it. The first top tip I have is not to rely on teachers to get you through your exams.