Herding dogs are barkers by nature, and we've got a
Pembroke Welsh Corgi and a Rough Collie, so we have lots of barking in our house. Ein especially is very loud, and barks at every sound, barks when Tuck and Colleen are playing, parks when Lucas tickles me. Barks when he hears the words "footsies", "Tucker feet", "bellies", and so on and so forth.
Nothing worked with getting him to quiet down. I have heard/read that one thing you can do is gently hold their snout (because most dogs don't like this) and say "hush" gently, but firmly while maintaining eye contact.
This did not work for Ein. Our approach to Ein's barking is not going to be a popular one here, but I don't have any regrets. We used a bark collar. We tell Ein "hush". If he keeps barking, we say, "Ein, do you need a bark collar?" If he still keeps barking, we ask him one more time "do you need a bark collar?". If he is still barking, we put the collar on.
Ein has only been shocked by the collar three times. That's all it took for him to learn what the word "bark collar" means and what it does. Now, most of the time, saying the word "bark collar" is enough to quiet him. If he doesn't quiet and we put the bark collar one him, he goes into a self-imposed time out. He knows that if he barks with it on, he gets shocked. So he lays down quietly on the floor until we take the collar off. I usually leave it on him for around 5 minutes or so. Just enough time for him to unwind and calm down.
Corgis are very bright dogs, so I don't know if this would be as effective (and less "shocking") with a chihuahua. I'm also not sure if a bark collar would be a good idea for such a small, delicate dog. But that is what we did.
Colleen doesn't bark as much as Ein does. Usually only if she sees something, or if the boys get her riled up. Telling her 'hush' (gently holding her snout, if neccessary) is usually effective for her.