Ever since we got the first dog, we have been doing this:
As soon as one start barking, we say "no" and "quiet". You have to be persistant and do it
every time and
as soon as they start barking.
This has meant that Kali hardly ever bark and if she does I can tell her "quiet" and she will stop and just growl instead.
The little one is a tougher banana, so now I do this: Every time she barks, I say "no" and "quiet" and go pick her up and hold my hand loosely around her mouth while I say "no".
This helps, but wether she will ever learn not to go off like a lunatic at the postman I don't know - her brain might be too small...