How people like Maskow remember tens of cubes, I have no idea. They must be androids or aliens.
I watched somebody do 12/13 Mbld at US Nats, and was amazed. Checked the results later and he only finished 8th. That's crazy. How do so many people do so many cubes?
How did you remember your pairs?
You weren't asking me, but I'll share anyway

. I still don't use letter pairs. Coming up with 400+ images is too much for me. Too many weird letter combos, and others letter pairs will get mixed up. I definitely recommend letter pairs if you are going to do Mbld or 4bld, but I think my method is easy enough for 3bld (still not recommending it though).
For edges, I don't use letters, I use the location of sticker (e.g. front right sticker is FR). That way there are only 22 images to come up with. I then pair 2 together and place them in different rooms of my house. An example lucky memo would start UF,RF,FD,FR,BU,FL...., that would be UFo on the RooF outside, FooD in FRidge in kitchen, BUg on FLoor in living room, etc. If I am doing multiple solves in a row, I will alternate that method with grouping 3 images together, so I'll have less chance of accidentally using previous memo.
For corners, I use letters, but I just memo them however makes some sense. When you get a vowel, you can make 'words' so it's easy unless you end up with all consonants. I'll put them in a pairs or groups of 3 if needed. Example scramble just now unfortunately yields all consonants VPRTSXW. I'd break it up VP RTS XW and just remember the letters and not make images. I memo corners last and solve first, so it's not hard to do.