Izzie is almost 4 months old and hasn't quite gotten used the harness yet. I bought the smallest size available and it's still a bit big. She likes to chew on it!
I put one of her little sweatshirts over the harness that has a hole through it. She seemed a lot more comfortable with the sweatshirt on. If your puppy is comfortable with clothes, maybe you could get her an outfit with a D-ring attached...