I was told that terriers were trained and bred to bark. That's how they alerted their owners to the rat they had cornered
I trained Eddie by shaking a soda can with pennies in it when I didn't want him to bark. It's negative reinforcement, but it seemed to work.
I read something else recently online that seemed to make sense. It says that if you train your dog to bark on command, you can train them to stop.
Here's the Web site followed by the excerpt on barking:
http://blogcritics.org/archives/2004/06/19/150954.php
"Barking is a job. Dogs are proud of their important work. It's part of their humble repayment plan for all this food and shelter.
"First, teach your dog to barkon command. Stand next to your dog and tap his paw with your foot. He'll look at his paw. You say, "Bark!" He looks up. You bark again; he thinks you're nuts. But he'll join you soon enough. And the moment he barks, give him a treat and say, "Thank you! That's enough." The dogs stops barking. He's eating. You're praising.
"OK? Now your dog is ready to go again. "Bark!" you command. No problem, he says. "Thank you" says you, handing over treat #2. "That's enough." Munch, munch and so forth.
"It's funny how fast my dogs have learned the significance of, "Thank you. That's enough." It'd work better if my arm were longer; they don't get a treat every time and it's time to work with them again, but even after just a few days there's been significant improvement. It works better than the squirt guns, though it's not as much fun."