How to stop your dog from barking at the door!

We invested in a bark collar that really helped tremendously. We have now trained my dog, who used to bark and anything and everything to only bark just to alert us that someone is at the door or there is a stranger that we do not want at the house. She may bark once when she hears a knock, but after the first knock, we will tell her to be quiet once and she will stop barking. The bark collar really trained her in about a week. It was amazing!