How to manage an aggressive dog?

At sitstay.com, it sells training vests that you can add patches such as "in training", "ask to pet me", etc. And I find it very useful when you want to keep people away in a polite way. I have a Newfie. It's hard enough to find a Newfie in the country side, imagine the reaction I got everytime we were in downtown Toronto. My dog likes interaction but he has a limit. If only one or two people approaching him and petting him, he will totally suck it up. Paw you, kiss you, etc. The moment the crowd start creeping up to more than three people and they all want to pet him, then he will react and bark. Having this vest, it will train people to ask you first. So you won't bump into situation that a group of people just rushing to your dog just because he looks friendly. Plus even with socialization from puppy stage, he never really likes children. He won't do anything when children walk by, but I know he is not one of those dogs that crave for children. So having this vest, you won't have parents without asking and just took their kids to pet the dog.