Dogs need company
Make sure that they have enough chances to spend time with human and/or animal companions so that they don't get lonely or bored.
Doggy playmates
If your dog is friendly towards other dogs, allow them to interact with them on a regular basis. Find out what to do when meeting an unfamiliar dog.
If you have more than one dog, house them together if possible, but make sure they have enough space and can get away from one another if they want to.
Make introductions carefully
Whether you're introducing dogs to each other or introducing dogs to cats.
If you go away, make sure you enlist someone responsible to look after your dog.
Avoid situations that scare them
Many dogs will either chase or be afraid of animals they don't know. If your dog is scared of, or aggressive towards, other dogs, avoid these situations and go to a vet or clinical animal behaviourist for advice.
Never leave your dog alone long enough for them to become distressed.