The short term memory can hold from 5-7 bits at a time before forgetting to learn in small bits. The best way is 20 minutes study with 5 minutes break in between with an activity like juggling/making a cuppa tea.
Go over class 5 minutes, 1 day, 1 week, 1 month and 6 months after class to get it into long term memory.
Association is key, so build in what you know.
Find a memory method that works for you and use that
Repeat parrot fashion
Smell and taste association works as well.
Have your study area as much like the exam as possible.
Revise in different places to make the brain know it's important to remember.
Get organised, set a time table for each subject, and remember use use spare time like travelling on the bus etc.