Here are some tips for those who have already adopted a shy guy.
Locate a qualified positive behavior professional who can work with you to help your dog become more comfortable in his world. Try the International Association of Animal Behavior Consultants (iaabc.org) or Truly Dog Friendly (trulydogfriendly.com) for good lists of positive behavior professionals.
Have a solid understanding of counter-conditioning and desensitization (CC&D), and make a strong commitment to practice this with your dog every single day.
Read about the Constructional Aggression Treatment (CAT) procedure and consult with your behavior professional to determine if this procedure might be a useful behavior modification tool in your dog’s program.
Teach your dog to target (to touch his nose to a designated target on cue). Dogs get confident about targeting (most love it!), and you can use the behavior to help him be more confident in situations where he’s mildly to moderately fearful.
Assertively protect your dog from unwanted advances by well-meaning strangers who want to pet your dog. You must not let people pet or harass him until he is socialized enough to tolerate petting and harassment.
Be prepared for heartache. Some poorly socialized dogs respond well to remedial socialization and grow into reasonably well socialized adult dogs. Others don’t.
For more details and advice on ways to socialize your puppy or dog, purchase Whole Dog Journal’s ebook Socializing Your Puppy or Dog: Making a Lifelong Difference.