I agree with those who recommend professional training. With our last dog, we took her to classes at PetSmart - a very good program. She was older than the "puppy" class, so we put her into one with older dogs, and she did just fine and socialized very easily.
Our current dog was 9 months when we got her and hadn't been socialized, so we have hired a trainer to come in and work one-on-one with her. As part of the training, we will take her to his business site (he also kennels dogs) and let her get used to being around other dogs. We've just started the program, and I'm liking it already. Since we had been able to teach her some of the basics like sit, down, stay, etc., the trainer can focus on more of the behavioral aspects of her training.