How do you deal with high energy terrier barking, getting excited about neighbours?

[Stu]

Straight forward question - how do you calm down a very high energy manchester terrier who gets up and starts barking at anything remotely "new" in his environment (environment being an apartment building in this case)?

As in, if he's lying down calm on a couch/floor, even half-asleep, tired after a walk, whatever, whenever - if he hears neighbours walking by, jingling their keys, opening the doors, the most remote sounds (& smells) at the end of the hallway, other side of the building, intercom phone ringing (when he now expects someone to walk through the door), etc - he'll jump up and start barking at the door...

I am assuming it's some sort of a mix of anxiety and protection of sorts, but have no idea how to deal with it anymore...

Suggestions?