Besides all the obvious such as health, movement, mentality aso. I keep the ones i think resembles the blood I have. For example. If I want dogs like the sire I pick the ones I think resembles the sire most when he was a pup. Or if it's a repeat breeding I'll keep the ones that resembles the best ones from the last litter the most. I also try to keep as many as possible from a litter as long as possible. If I would buy a pup from a breeder his word weighs the most. As I wouldn't buy a dog from a breeder that haven't a good knowledge of his dogs. If I for example would buy a pup from Jack, his word and his knowledge would weigh in the most, as nobody knows his dogs better than he does.