Preventing aggression

The primary goal is simple -- never allow any dog to achieve dominant status over any adult or child. If dogs always know their social ranking and are never allowed to challenge people, they will usually be good family members.
The first rule for preventing problems is to match the right breed and puppy to the right owner. In other words, the Rottweiler or Akita is not a suitable breed for a meek or mild owner or the macho owner looking for a tough, aggressive dog; the Dalmatian and the Flat-Coated Retriever do not fit sedentary lifestyles; the Shetland Sheepdog or the Chihuahua do not like boisterous, rowdy children, etc. Likewise, the litter bully will take over the home of a submissive owner and the shy puppy needs extra attention to adjust to an active household.
Puppy testing done by the breeder can help. The test includes social attraction, following, restraint, social dominance and elevation dominance.
Aggression prevention includes early socialization. Puppies should be handled gently, especially between three and four months of age. They should be hand-fed by children and adults and taught to take food without grabbing or lunging. They should not be allowed to chase children or joggers, jump on people, mount legs, or growl for any reason. They should never receive or be part of rough, aggressive play such as hand-fighting, wrestling, or tug-of-war games. Puppies should never be physically punished for aggressive behavior; instead, they should be denied the rewards of aggression, restrained from repeating the infraction, and taught alternative behavior.
If puppies bite at or jump on children, the children should take charge by screaming "Off!" and crossing their arms (to protect hands and arms from being grabbed) and turning away. Puppies love to play; if fun is denied when they get too rough, they will learn to play more calmly.
Puppy parties, where children of all ages visit and play gentle games and offer food rewards are helpful for the children and the puppy.
The puppy should be part of the family pack and should learn to accept delivery people, repairmen, and other strangers. Once they have been vaccinated against the common canine diseases, puppies should be exposed to non-aggressive dogs so they learn that other dogs as well as other people are friendly.