This is a tricky one to fix and that's because you don't want the dog to stop barking at all, especially if they're going to bark if you have an intruder. You do want to be able to stop the dog barking when you tell it to, though.
First, let's examine a couple of things you should not do:
Shout at the dog : You should never shout at the dog; this basically gives the signal to her that you are reinforcing the behavior by joining in.
Give her affection : Your dog will see this as praise for barking, which is not what you want.
Now, let's look at a couple that you can do:
- Teach him a "stop barking" command : teach him to bark when you say "Speak!", then command him to be quiet. After a couple seconds of the dog being quiet, praise him with words or a treat until he learns this routine. Then practice ringing the doorbell yourself and using the command with treats until the dog actually stops barking on demand.
- Distract the dog : In some cases, there has to be some distraction in order for the dog to stop barking. The most effective form of distraction is something that causes a displeasing sound to the dog (like shaking a tin can with pebbles / coins inside of it). After a while, he'll associate that awful sound with barking at the doorbell, and so will refrain from doing so.
For more information, see the legendary Cesar Milan's full article on preventing dog barking.