sheltie can't stop barking

[ozzys mom]

We have three dogs, one of which is a ten year old sheltie. The past year or so he has started barking much more often. He barks at anything - the obvious (door, phone) and the not so obvious (me calling to my husband in the other room, one of us standing up from the couch). He barks when we leave the house. He barks the whole time he is on a walk. We have tried praising him when he doesn't bark, yelling at him (NO!) when he does, citronella collars and shock collars. He KNOWS it is wrong - as soon as he barks, he drops to the ground and flinches as if he is going to be spanked. It is almost as if he can't control it. The collars would startle him, but two minutes later he is back at it.

Anyone have any other ideas? We are expecting a baby this year, plus I am sick of the dog looking guilty and feeling bad.