How do you stop dogs from barking?

I have never tried it myself but my dog trainer friend told me how to teach a Dobermann not to bark. He said to tie up its muzzle with a soft cloth in the evenings because that's when they are likely to bark. Not for too long. Just about an hour each time. The Dobermann would soon learn to be quiet. Having a Dobermann which doesn't bark can be asset because Dobermanns are bred to be personal bodyguard dogs.