I was chatting to a behaviourist the other day (like you do . . .) as Parker has taken to barking at the sound of a leaf falling in the next village

She said immediately "Pet Corrector Spray" and produced a can from her pocket. It is about £5 for a small bright red can of noise or £10 for a big one. She said to let the dog let you know someone was at the door and then you say your word to make her stop ie shush or quiet and give a quick squirt of the spray which
should interrupt the behaviour at which point you praise like mad.
Make sure you keep the can with you all the time to correct her
each time she starts with lots of praise. I have only given about three bursts of the spray (boring, it's quite fun

) as all I have to do is show Parker the red can and he stops when I say "shush".
The alternative is to teach her the Dunbar method which involves teaching her to 'speak' and once you can get her to bark when there is no reason to bark it is easier to get her to 'shush' without you having to worry about who is knocking at your door. Have a look here
http://www.dogstardaily.com/training/woofshush for a better explanation.
Good luck - it is worth the hard work - for your ears' sake, if nothing else!