I like the pennies in the can method, but we had to use a heavy tin can rather than aluminum can as it made more noise. The sound should be loud enough to get their attention. Screaming no only agitates them further and causes them to bark more, so the pennies work by getting their attention and saying, "no" or "shhhh", and I found putting my finger up to my lips in a shushing fashion helps. This requires training and just because you do it once or twice they won't immediately stop barking. If they aren't even paying attention to you, increase the noise factor. You have to ALWAYS correct one more time then they bark, you can never let them have the last bark, or next time they will be more aggressive, insisting on the last bark. Before you attempt this method you have to promise yourself, you will keep cans available, we had them in three rooms, and you will always use them, and you will always have the last shake. I'm confused when people say it doesn't work, because the first time you do it, they may bark 50 times, but if you continue to do it, they will bark less and less. If I just pick up the can, Joey will stop barking, and many times I can use put my finger to my lips.
I don't like the spray method because I think it's easier to teach when the cues are similar, in other words, the noise of the bark, and the noise of the coins are similar. Also, when a dog is sprayed, he connects that with you rather than the barking, and if you aren't around the barking may increase.
I agree with those of you who have suggested thanking for barking, and when Joey was a puppy and barked at night, I would carry him around and check things, and tell him thank you, and "it's ok". For late night barking or thunder storm barking I can just say, it's ok, and he goes back to sleep, but for frivolous barking as when he sees someone through the window in the park, I use the coins in the can. I also taught him to ring a bell so that I'll open the door and he can go outside and bark at the dogs, I want him to have some fun.
To decrease barking at guests, I just give them a piece of his kibble to give him, and he usually accepts them. With the can method you do have to have periodic renewal training. Barking is an ingrained behavior and you cannot eliminate it, your goal is to stop it, when it's not welcome.