Early socialization is a key ingredient to preventing fear aggression, as well as a proper breeding line of dogs. Fearful parents produce fearful pups. Boldness is inherited to some degree. By socializing your dog as much as possible, you are showing him not to become afraid, no matter the circumstances. This entails taking your young dog with you into lots of different environments and exposing him to lots of different people, dogs, noises, sights, etc.
Familiarity produces calmness.
The first time you jump out of an airplane at 5,000 feet, you're petrified. By the 500th time, it becomes blase' and old hat. The first stranger your dog meets is going to be scary. Once the dog is a bit older, the method is the same but the approach is slightly different. Obviously, the more nice people your dog meets, the more he will begin to see strangers as nice folks.