Socializing your dog should be a life long process. You want your dog to be exposed to as many people, pets and situations as possible so he learns how to behave and react to different things. Take him to new places every few days or so and introduce him to family and friends.
Introducing To Other Pets
If your dog has been socialized growing up, he shouldn't have problems meeting other dogs. But you still want to take it slow and see how things go before letting him loose. The best way to start is by introducing your do to other dogs that you know are friendly and socialized. When introducing your dog to new dog friends, keep him on his leash so he can sniff the other dog, and you can reel him in if you need to. Try to keep the leash slack; if you pull too hard, you can trigger his "fight or flight" instinct. Always introduce dogs on neutral turf, so neither of them feels threatened.
How You Can Help
Dogs pick up on our attitudes and feelings. If you feel stressed, your dog will notice and react to it. If you sense a potential problem, try not to tense up or give your dog reason to panic.
Another tip is to divert his attention. Many dogs are hams and love to please their families, so ask your dog to sit, or give you his paw, or some other easy trick he has mastered. Give him lost of praise and attention (and treats if you have some!).
Set Your Dog Up For Success
If you don't know how well your dog has been socialized, always go slow and set your dog up for success. For example, don't take your dog to a park with lots of pets and people if you don't know how he will react. Start slowly, with one or two other people and one or two other dogs, for just a few minutes. Don't take your dog for a car ride only when going to the vet or dropping him off at a boarding facility. He will soon learn that car trips aren't always pleasant. Take him for a ride to get gas in your car, for example, or the drive through ATM. Your dog gets to stay with you and you don't have to worry about leaving him alone in the car.
Whether you take your dog to the park or for a walk around the neighborhood exercise keeps your dog's mind and body healthy. He can meet other people and dogs and burn off some energy. Less pent up energy means less distraction from you and overall, better behavior.