Mikkel Becker with VetStreet.com explains how to teach your pooch to be quiet.
"So one of my favorite ways to teach this is to actually teach the dog to bark in the first place. What you do is to tell the dog to 'speak' or have a certain word such as 'bark', whatever you want to use, wait a second or two and then either knock on the door behind you or have someone ring the doorbell. And usually that starts this chorus of barking from your dogs. And you can get excited, ‘OH, what a good dog! Good bark; nice bark!''
Once your dog learns to speak on command, you can teach the quiet command.
"You want to tell them 'quiet' or 'shush' and then wait a second and pull out a toy or a treat or something that they really enjoy. And as soon as they are quiet, then they get that treat. That way, quiet is the most rewarding part of the whole deal."
Mikkel says once your dog gets the hang of it, practice the quiet command in different parts of the house.
More Info: Training video: Stop Barking at the Doorbell