My dog used to be a big time barker (not at all times, but in the car and park where she would see other dogs).
What REALLY worked for me was a water-spray bottle. I had to put a little vinegar in the water before it fazed her- I read about doing this in a dog book. The vinegar won't hurt your dog, but they do not like the taste,etc. Also try and squirt them on the sly, so they don't really know it's you, and do it immediatly after they start barking. Then when they are quiet, praise them for that. Now, all she has to do is see the bottle, and she calms down. I don't even carry it with me most of the time anymore.
I agree with everyone else about not be-barking or no shock collars.