Our Shih Tzu will throw up if he doesn't eat much during the day, or forgets to. He will basically throw up yellow bile and we encourage him to eat or give him a biscuit. He doesn't throw up too much though. If you think your dog is thirsty and for some reason isn't going to the water dish, give him/her an icecube.
Mine does the same thing, when she doesn't eat much the day before she wakes up the next morning and does this. I've also had to change her food to a special food from the Vet's office with dogs with digestive problems and this has helped.