I have two yappy Chihuahuas and here is what I did: It's much more effective to set up practice situations than to wait for real life situations. First things first: If there has not been ample socialization, exercise and mental stimulation, training will be a losing battle.
Increase exercise, physical and mental and more socializing.
THE SEQUENCE:
You cue "Bark!"
(get something to make the dog bark...door bell)
Dog barks
You praise: "Good!" after a few barks
You cue "quiet"
You show the dog the treat (to prompt quiet)
The dog's eventual distraction from barking by the treat
3 to 5 seconds of quiet during which you praise: "sooooo gooood."
You furnish treat after the 3-5 seconds of perfect quiet
Repeat with less and less visibility of the treat prompt (you still furnish after perfect quiet)
Practice with pretend visitors a few times.
Do it over and over until the dog knows the game. It may take a few sessions so hang in there. How do you judge whether he knows the game? He knows the game when he barks on the cue and doesn't need the door bell anymore and he quiets on the first quiet cue without having to be shown the treat. You still give him one from your pocket or from the cupboard if he quiets on cue, you just don't show it up front anymore. If he ever interrupts the quiet with a muffled bark, give him a no reward marker such as "Oh! Too bad" and start counting the quiet time from the beginning again. He has to know that barking during the quiet time was a mistake that cost him his treat.
Practice later in various locations and situations. Get good at turning on and off his bark and gradually increase the quiet duration. You have to acquire the ability to turn this on and off, back and forth in different contexts. Lots of practice with different friends and dogs in sight, but never moving ahead faster than he is able at a lower distraction situation. When you can turn his barking on and off anytime, any place, you have conditioned a "muscle." The more practice, the stronger the “muscle.†Then you can go to applying this to real life situations. You may have to go back to prompting with a treat again at first. Keep treats handy.
The hardest thing about bark training is that the first few tries it may seem to be futile, but once you get over the hump, it's amazing how fast it will go. Stay with it. (most people never get over the hump)
In addition, teach a good down/stay to go along with the quiet. Some dogs can't seem to bark while in that position.
With extreme, persistent barkers, if he likes a tennis ball, teach him to go fetch and carry his ball every time there is a trigger that causes him to bark. He can't bark very well with a tennis ball in his mouth.
Finally, you can teach the dog that barking after being told to quiet earns him an instant time out away from the action. Dogs bark and crowd up against the door because they want to be where the action is. Most dogs find the door goings on as sufficiently fascinating and it really bothers them to be banished to a back room. Timing, as usual is very important. Once you've issued the "quiet" cue, the very next bark needs to meet you instant "ooh! Too bad for you" and quick escorting to the penalty box. A minute or two is plenty in the penalty box. Don't release him until he has been quiet for at least several seconds. Timeouts for watchdog barking can elicit a watchdog request barking..."let me out of here!" Don't panic. Wait for your five to ten seconds of quiet.
If your dog has a very low threshold and goes off at the slightest noises and other changes in the environment, it helps to get him better habituated. Take him out more and invite the world in to visit more often and associate these things with something the dog loves. Expose him to a wider range of sights and sounds.
