How do you train a dog to bark at strangers?

Most people would say a behaviourist.

However, one of the dogs I take out had severe aggression with other dogs, like yours has with humans.

Now, Cesar Millan has a method called Calm Assertive energy and she's getting so much better now!

Basically keep your calm and ignore the person, and she should get it into her head that it's not an issue with her. Once the person is past sit her down and wait until she is calm and only praise her when she didn't bark.

It takes patience but you do get there.

Obviously when the person passing see's an aggressive person their going to cross the road or something and the dog then feels dominant towards them.

Instead you go across the road so the person feels better and your dog will too.