Hi there. My name is XXXXX XXXXX I'm happy to help you with your question about the beautiful Bella today.
As I'm sure you already know, yelling at the dog to be quiet won't work because dogs who are already barking, just interpret our yelling as an attempt to join the conversation (for example, if the dog is barking at the mailman, she's saying, "Hey! Owner! Come see who's outside!! Is this a friend? Is it an enemy?? What should I be doing right now?? Oh, you're barking loud too, so I should probably keep barking!!!"). Any yelling we do can accidentally encourage them to keep barking. Luckily, you're already on the same page as me with not yelling while she's barking.
We're going to need to train her to act differently when she's barking.
When your girl is barking, you need to ignore her. Wait for her to be quiet, and after a few seconds of silence, start using a key-work like "hush" and then giving her a treat. Make sure you do this immediately...timing is super important, but make sure you don't give her a treat until she's been silent for a few seconds. Try thinking like your dog...would you rather keep making noise, or be quiet and get a yummy treat?
Do this training several times daily until your dog has figured out that the key word means it's time to be quiet, and that being silent brings treats, but that making noise doesn't get her anything. Over time, you're going to make the time between when she's barking and her quiet longer and longer. After a few weeks of training, you should be able to use the key word when she's actually barking and then giving him the reward when she stops.
We're also going to need to get your girl on a strict regimen of exercise. I understand that you have a busy life and things to do, and that you already do give her a good amount of exercise, but working breeds like herd/guarding dogs NEED to be exercised every single day, and I'm talking more than a quick walk around the block. A youngster like yours needs a couple miles or more a day to drain her energy and keep her from getting into trouble because she's too wound up. If you're unable to walk her yourself, you might want to consider hiring a pet walker. In my neighborhood, I live close enough to the colleges that I put up fliers at the schools and hired some of the track runners to take my dogs for a run on the days I can't. I pay them $10-$15 bucks a day, and it really, REALLY works. The dogs get the exercise they crave, and the students get a couple bucks in their pockets for doing something they would have done anyway (the running). Several of my runners say that they feel safer running some of the woodsy trails around here because the dogs 'look' scary enough to ward off anybody who might bother them (luckily the strangers don't know that my dogs are all big marshmallows!!).
Investing in a backpack for her to wear on the walk will also help. It can carry your keys, water for you both, poop bags, whatever you like...the added weight will help tire her out faster than if she was just walking without it.
Also, you might want to consider clicker training your dog. This involves buying a 'clicker' which has a little metal plate in it. You press the plate and the thing clicks, then you give the dog a treat immediately. Soon, the dog learns that if she focuses on you when she hears the click, then she'll get a treat. It can often help when trying to get them to pay attention to you, rather than what is outide that window. You can see more about clicker training here:
I know you don't want to use a shock collar with her, and I tend to agree with that kind of thinking...however, we could try a new product on the market called InteroStop. I've found that it's a nice product that seems to work well. It essentially combines a pheromone with a noise...you can read more about it and even buy it here: http://www.amazon.com/InteroStop-InteroSTOP-Noise-Interomone-Spray/dp/B009IUQMS2
As you're training, It's important to remember to not repeat the key word over and over, since this can actually encourage her to keep barking rather than being quiet.
I also like using a back-up plan: diversion. Instead of using the 'hush' cue when she's barking, you can call her over to you and ask her to perform a desired trick like sitting or fetching a toy. Obviously, you don't want her to act like a mute if someone were to be bothering your house...it's good that she's letting you know someone's around...but by making sure to reward her being quiet, rather than her barking, you'll end up with a better behaved dog, and less stress for you worrying about her incessant noise making.
And please, please, PLEASE remember that this behavior didn't start overnight, and as such, you won't be able to break him of this habit in a day or two. Patience (and maybe a pair of ear-plugs) here is the key! It absolutely sounds like you've got a great start on this...but with a few tips and tweaks, I have no doubt we can get her behavior stopped.
I hope this helps.