I think the only times I've had to really work on associations were with LuLu and Lu-Lu and with Sockstickler and Sockstiklish(I'm not 100% sure those are the accurate latter duos names, but it's close enough). With the LuLu/lu-Lu's, it was soon readily apparent that they each had their own unique style, and when I figured that out, it got easier to distinguish the two.
I still have trouble with the later duo, and usually look to their genders to figure out who is who, as one's female, and the other's male.