They are pretty ravenous about meat, so when they grab at my fingers pretty roughly with their teeth, I fold back my middle finger under my thumb and flick them under the chin to startle them. I praise them very soothingly and allow them to eat. I allow NO 'rough and rile 'em up' games from visitors as the pups are growing. I also require puppy and basic obedience classes of my puppy buyers. During that time, if they bite a littermate, it bites them back or stops playing with it and goes away.
Human flesh is off limits.
I want to inhibit biting. These people may want their pet to be a watchdog, but I tell them a biting puppy is not a protection dog, it is a Lawsuit on four paws. So for the puppy that bites in response to being held or because someone tried to walk past their food bowl etc., I teach the 'scruff and cuff'. My right hand sneaks up under the jaw and cuffs it. This is done with an open hand and stiffened fingers, enough to cause a submissive reaction.