After you have had success at socializing your pet to other dogs, cats, etc, don’t forget to continue offering new experiences and reinforcing the current. We tend to spend time teaching a new behavior and then stop using it such as obedience training if we are not taking them to the vet for a while like we were in the puppy stage. This can also be a problem with car rides or being around other dogs or animals.
Once a month is a good frequency to keep unused behavior remembered. A car ride to the store can be an easy way to help your pet not be fearful of a trip to town (AKA the vet).