Like Wolf, I have two Black Labs. I got them as young pups a short time prior to getting my chicks. After the chicks were big enough to come out of the box I kept in the coop for them and free range I was afraid I would have a problem.
For about a week I took the dogs leashed out to the coop and sat them down at my feet. Like mentioned by Wolf, if they looked twice at the chickens roaming around us I sternly told them"Those are my chickens!" The following week I took them out unleashed, laid them at my feet and performed the same ritual. By the end of the week they would lay at my feet and the chickens actually walked over the dogs, and they just laid there. After that I let the dogs out to run around with the chickens free roaming, and watched from inside. If I saw them them even go in a chickens direction, I would holler "Mine! Not yours!" One time since then I saw one of the dogs, which are now 6 mo. old, chasing one of my EE roosters. He pushed it to the ground with his nose and just held it there. I think he was just curious and playing. Regardless, in a matter of seconds I was out the door, and on top of the dog. I grabbed him by the scruff, flopped him to the ground and just railled on him verbally for about a minute. When I let him up he high-tailed it to his kennel in the house and did not come out for half an hour. I had to coax him out. Since then I have had no problems, except trying to keep them out of the coop, cause they like to sample the chicken feed.
As I stated, I think he was just playing, cause he had ample time to kill it even though I was there pretty fast.
This being said I think one should realize that no matter how good you train a dog, there is a chance of it killing a chicken.
I have decided that I want open spaces for my dogs and my chickens. Hopefully I will not have to pick at some point which I will restrict to what I feel is an unnatural enviroment, i.e being pinned up 24/7.