My 1 year old terrier just won't stop! He is driving us nuts. He is an angel in the house and on a walk. However, in the garden he can hear the planes overhead, the neighbours' dogs, people walking by, motorbikes - anything. Basically, he is such a nuisance in the garden that we have to bring him inside.
We say, "shut up", "shh", "no", and take him inside when he barks too much but he doesnt get it.
The only time he is quiet outside is when he is chewing a bone.
As strange as this seems you have to teach him to bark on command in order to teach him to be quiet. One of my dogs is a barker and he now 'speaks' as soon as he learnt this i could teach 'zip it' so now i have control to when and how long he barks for. (the other dog is mute so no problems with him!).
You have to work just on the barking on command first, treat and reward this, and then when, and only, when he is totally secure in this you can teach him the quiet. You wait for a 3 second break in the barking and then just treat (and click if you use a clicker). Eventually you can add in a word and a hand signal. Then you slowly withdraw the treats adn just use verbal/hand commands.
I'd also say it's possible he may be looking for excitement. You might want to step up the level of exercise he gets, and the amount of training you are doing with him in the house. Terriers are happiest when they are busy - clicker training would be perfect for this - MoreThan's suggestion above is perfect but think about other commands you can teach him. You could also have fun with him in the garden doing "treasure hunts" with Kongs and so on - again the emphasis is on keeping him busy and occupied.
Snap! I posted a question about this just the other day about my 1 year old terrier! I tried teaching bark and quiet simultaneously as per the instructions on the Karen Pryor clicker training website but got myself too confused In the end we spent 15 minutes the other evening winding him with all his usual triggers and then clicking and treating every time he was quiet.
To my surprise he has been much, much better ever since! We've got a pot of treats in a handy place by the front door and every time he barks I rush to him and give him a treat as soon as he's quiet. He now barks just once when something excites him and then comes to us for a treat. The neighbours have remarked on how quickly he's improved. I know it's early days but he's like a different dog already!
The other thing we've done is to totally stop telling him to be quiet for now, whereas previously we'd been saying 'shush', 'quiet', 'stop it' etc. In time we will re=introduce 'quiet' as a command but for now we're trying to keep totally calm when he barks and just click and treat as soon as he's quiet.