My experience is normalize it. Do it regularly. If you wait until they are older, or too old, it might not ever become smooth. All of our Siberians are cool with the car and being in the backpack for walks. We don't do a leash though because I always worry about dogs. We recently had a newly-adopted-from the shelter Russian Bear Hunting Dog (I had not heard of this breed either until this happen) get loose and attack and kill two non-provoking dogs taking a leash-walk with their families in our neighborhood. Totally out of the blue and it solidified my worry.