Memorising is an art.
But to have it in your head such that you can never get rid of it is a trick.
One of the easiest ways to do it is to make notes.
1. Read what you are supposed to learn. Simply read and understand.
2. Take down pointers - Key words that help you connect the dots and remember the story whenever you read it.
3. Read the required part again.
4. Revise with the notes whenever needed.
This gives your brain a memory of what you had studied in less time and effort.