I’m pretty sure I didn’t come up with this and other people use this technique, but recently I have been using a pretty neat system for memorizing numbers that I also use for words and cards as well. It will take practice in the beginning, but as your speed increases you will find that the retention with this technique is much more effective than traditional methods (for me at least)
Here is the technique:
Have a memory palace
Have a designated person in each loci. (Set this up in advance)
Use the major system, where each 2-digit number represents a word
Use the people in each of your loci to group 3-4 words in one cluster.
Here’s an example:
Say you need to remember 77 (cake), 25 (nail), 36 (match), 20 (nose).
And lets say the first person in your first loci is Albus Dumbledore.
Heres how I would group these words together: Albus Dumbledore is standing at my front door eating CAKE (77). As he bites into the cake, he realizes he just bit into a rusty NAIL (25) and is in great pain! To distract himself from this pain, he pulls out a MATCH (36), lights it, and shoves it up his NOSE (25).
This is just an example that I would personally use, but hopefully you get the idea.
What you have done here is effectively stored 8 digits in one single loci. With practice, you will be able to do this very quickly and efficiently.
I hope all of this made sense, and I hope it helps!
Cheers, and best of luck!