Part of it is organized, like you mentioned in your example. Especially for the persons I try to have an association with the number for most. For Hanuman I always see the image on the clouds with the sun behind him, so the association with the sun made him 0. Brahma has three heads, and thus an association with 3.
But to learn it, I have another method. Lets take number 679, the Grim Reaper and a Lysosome, as an example. A little background, the lysosome is reprisented as a close up textbook-view of a lipid bilayer with a membrame, thus all the little spheres outside and the tails inside. Their job is to “eat” the cell’s waste.
Now, the number 679 is split into two parts, 67 and 79. Back in my old PAO, 67 would be Han Solo pilotting the Millennium Falcon and 79 would be a Rancor getting crushed by a gate. Important are the action “Pilotting” (67) and the object Gate (79)
I see the Grim Reaper (679-P) Pilotting (67), goggles and headset included, a heavy metal Gate (79), like the ones you see in medival castles. He crashes into a Lysosome (679-O) in an attempt to break it open, but the Lysosome ends up eating him.
The actions are the same as my original PAO, so those I already know. So in the end I have my Person, doing an Action with a 2-digit-Object to a 3-digit-Object. If I want to know 679, I just have to ask myself “Who was pilotting the gate, and what did he do?”