The best way to memorize long answers is to understand the answer first, break it into key points or keywords, and then recall those points instead of cramming the answer word for word. Your brain remembers a structure of points far more easily than a wall of text, and in the exam you expand each point into sentences.
Understand before you memorise. Read the answer and make sure you actually understand it. Memorising something you do not understand is slow and fragile; understanding makes the points stick naturally.
Break it into points. Convert a long paragraph into 4 to 6 key points or keywords. Memorise the points, often with the first letters or a short acronym, and in the exam write each point and explain it in your own words.
Use active recall. Do not just re-read the answer. Cover it, try to write the points from memory, then check. The struggle to recall is exactly what builds strong memory.
Revise with spacing. Review the answer after a day, then a few days later. Spaced revision moves long answers into long-term memory far better than one long cramming session.
Example: A Class 12 student in Lucknow turned a one-page history answer into six keywords, learned the keywords, and practised writing the full answer from them. In the exam she reproduced it easily by expanding each point.
One practical tip: Practise writing the answer from just your points, under time, a couple of times. By exam day your hand and memory know the flow, and you will not blank out even under pressure.