Any multiples of nine (so nine, 18, 27, 36, etc).
It's come from years of collecting trading cards...the album pages generally have nine slots for cards and when I'm counting along for where a card should go, I think of how far it is from the nearest multiple of nine. Like...if I'm looking for #31, I know it will be the fourth slot on the appropriate page.
All the numbers are just drilled into my brain now, so I always go for something like 36 or 45 if I need a random number.