Take them places, especially walking in the park or in your neighborhood if possible. We can't walk through our neighborhood because we have no sidewalks, and I think it would be dangerous. But, we take them to the park. We took Jake camping last year, that's a great way to socialize them. We have neighbors who have dogs, so we let them play together (ALWAYS closely supervised). The trouble is, no one really has a dog their size other than my sister who has a maltese. With her schedule, we haven't been able to get the dogs together, but we'll work on that. I take them to places they can go, like the pet store. We take them when we visit people. They LOVE people and aren't afraid of other dogs.
We started out gradually so they didn't get overwhelmed.