the citronella collars usually work really well. My sister had a problem barking dog who came to visit us every so often and the citronella collar was the only thing that really helped. Just make sure you fill it up, point it in the right direction, turn it on and do NOT let the battery run out!!!

Other alternatives could include some behavioural training, places like Bark Busters send someone out to retrain the animal - not sure how pricey it would be if you have a very resistent barker though.

Maybe he feels insecure in the yard? You could try some measures like blocking his line of sight into the street, repositioning his kennel so it's close to the back door, make it really cosy inside, etc. Also keeping them entertained when you're at home (walks, playing, kongs, bones when you're not there) do a lot to keep them settled wehn you're not there. Good luck