I was trying to keep the blockish square heads, low ears, no pointy noses, correct sized eyes for the head, etc.
There's a lot that goes into the hold back process, it takes multiple generations to correct flaws, so holding back the best of the best is important.

Holding back but not becoming attached too much since even ones you think at first might be primo examples may not be once they mature into young adults.

I always picked them out as they fuzz up and then I would start weeding out the non hold-backs so by the time the litter hits 3 weeks old the mom generally is only caring for no more than 5 kits.
By the time they are weaned they tend to be a bit more developed that way versus leaving all 10+ kits with the mom.