How can I make my memorization very strong so it doesn’t fade quickly? And is it true that repeating a surah many times during memorization strengthens it, or should the repetition be after finishing the memorization?"
You may find this article helpful for making images/data stick longer.
As for spaced repetition,I personally use memory technique to memorise first,then I review using active recall on the same day,then the next,then the next 3d,then the next week etc
Say,I learnt that the GDP contribution of the Agricultural sector is 14.39%
Then, for active recall, will simply ask myself what the GDP contribution of the Agricultural sector is,which would lead me to an image and that would lead me to another image which represents the number 14.39 for me.
For me this is, A donkey(for GDP,the word for donkey in my language is ‘Gadha’;to be on a donkey is ‘Gadhe par’,which reminds me of GDP)then the image of a donkey flashes,I see who is sitting on the donkey,and lo and behold! It is a facial cream called ‘DerMa-B’.
I use major system for encoding numbers.
So for me derma-b is
D 1
R 4
M 3
B 9
14.39!
Thank you very much