How can I help my dog be less nervous around strangers?

My puppy is about 8 months old, and is really wary of strangers. When a new person comes in he stands four or five feet away and bark at them, and refuses to go near them for a couple of hours. Neither of our other dogs are like that. I know I shouldn't tell him it's okay and coddle him, because that's reinforcing the behavior, and I don't want to add to his stress by snapping 'No' at him, so how can I correct this behavior and help him feel more at ease with strangers?