Barking at Strangers
Let's talk about barking at strangers. First, rule out fear-aggression with a vet behavior consult, because what looks like guarding can be terror in a fur coat. Once you know the dog is confident, the fix is counter-conditioning, not correction.
Step one: every time a stranger appears at a distance where the dog notices but does not bark, mark and treat. That is called the look-at-that game, and Karen Overall of the ACVB uses it as a foundation exercise. Step two: never grab the collar.
Grabbing the collar rewards the bark with contact. Step three: place the dog on a mat ten feet from the door. Step four: train the dog for your family and your neighbors, not just yourself.
The mailman does not care about your training schedule. Now, the deeper fix: the dog is rehearsing the wrong thing every time the doorbell rings. Counter-conditioning replaces the rehearsal with a different one.
Now here's Steve's notes.