You kind of have to think like a dog.....

You are part of a pack..... You have to make sure that he is 'below' you. So when he barks.... If you quickly tell him to stop it, then he kind of just thinks you're joining in.... Sometimes IMO the best thing to do is ignore behaviour you don't want.... Then there is no incentive to continue it.

Sometimes you're going to want your dog to alert you to things.... But if you don't react to the things like barking as possums, birds, or people walking past.... Then he might realise that there is no reason to do it, coz all is well.... You're in control.

It's really hard to explain.... Don't want to sound like the dog whisperer or anything, but this is how I treat my dog and he is the most relaxed mountain of fur going around!

Hope it gets better for you.