But this advice is for "normal" dogs. Those puppies who are curious, brave, open minded, outgoing. We take advantage of that open mindedness while they are young. If they aren't exposed to all sorts of things when they are young, they can become suspicious of strange things when they are older.
But for puppies, adolescents, or adults who are already displaying fearful tendencies, it's too late for typical socialization. Actually, not only is it too late, it can often be detrimental. It can turn a shy dog more shy. It can even turn a shy dog into an aggressive dog. And just like in humans, continual stress can even manifest into physical/health problems.
Attempting to "socialize" a shy dog is actually "flooding" the shy dog.
Example. Let's say a child has never been introduced to a swimming pool or lake or pond. The parent decides that the quickest way to teach the child to swim would be to throw the child into the deep end. What could result from this:
1. They child might flounder around and learn to swim
2. The child might drown
3. The child might never trust his/her parents to protect him/her
4. The child might develop an unnatural fear of all bodies of water.
5. The child might develop any number of behavior issues that might or might not appear to be related to that particular event
Back to dogs. Take a brave young puppy to a pet store and he might have a great time meeting all sorts of people, experiencing all sorts of smells.
Take a shy dog to pet store. He's afraid just being there. Wants to leave but is not allowed. Not only can't he leave, scary people start approaching him and he is trapped on a leash. The pet parent says to the dog "it's okay" but the dog is thinking, why is my human forcing me to do this? Why isn't he protecting me? I guess I'll have to protect myself.
So the dog tries her best to communicate that she wants to be left alone - head turns, eye blinks, tongue flicks etc.. No one notices the communication.
So the dog tries harder to protect herself. She might try to hide behind her human. But the human says the dog needs to learn to cope and pushes the dog towards the strangers. Seeing that her human isn't going to protect her, the dog barks at the stranger. The stranger doesn't leave, the human doesn't protect the dog. So the dog realizes that they only way to save himself is to growl or even bite.
The human, seeing that the dog is upset around people engages in more flooding - assuming the dog just needs more socialization. The dog, remembering that all the subtle hints didn't work last time, goes straight to growling or barking at the next stranger who approaches.
The human, believing the dog needs even more socialization continues taking the dog out and about to meet people. Hoping that the dog will eventually get used to scary people (or scary dogs, or scary noises, etc..) Now the dog learns that he has to be proactive. Get the humans away long before they start bothering him. So the dog starts barking and growling at people when they are 50 feet away...
Back to the human child and swimming. Although tossing a child into the deep end might work for a few children, a better way to teach a child would be to start at a place where the child isn't scared then work your way up. Maybe let the child wade in the shallow end. Then let the child dunk her head for a couple of seconds. Then let the child wade in slightly deeper water while wearing floaties.
Back to dogs.
Our first priority when we live with a shy dog should be safety and security. And we need to understand that the dog might not ever be a therapy dog or an agility champion, etc..
Next we can start desensitizing the dog to things that are important. Like going outside to potty or walks. So our first trip outside shouldn't be a 5 mile hike (just like we don't toss the human child in the deep end of the lake). We might need to go out for 2 feet then turn back around.
We need to work on one scary thing at a time. So while we are working on being outdoors, we shouldn't also talk to strangers while we are outdoors.
While we are building up positive associations to riding in a car, we shouldn't take trips to the scary pet store.
For more info on helping scared dogs, see
Leslie McDevitt's Control Unleashed books and DVDs and her Pattern Games DVD
Kathy Sdao's "Does Pavlov Ring a Bell " - excellent info on counter conditioning and desensitization
Ali Brown's "Scardey Dog"
Suzanne Clothier's videos: http://www.youtube.com/user/
For info on recognizing stress in dogs, see Turid Rugaas "On Talking Terms with Dogs: Calming Signals"