There was an episode of the Dog Whisperer that covered this topic. Perhaps you can do a search for it.
Basically - if you dog views you as the true leader - then you can "claim" your chickens as your property and let your dog know not to touch them. It's hard to explain - but, it works for Cesar Milan (The Dog Whisperer - on the National Geographic channel).
I had a border collie/newfoundland cross that I trained with a pinch collar. After he understood he wasn't to hurt them, he became the herd protector of the chickens that his genetics were geared for. Depending on your dog's breed you may or may not be able to direct his instincts. I was able to free roam my chickens without predation when I had that dog---I miss him. (old age caught up with him). I now had to build a bigger run to prevent predation.
I have a Jack Russell, and he wants to/does kill ANY little(or big) furry animal he can.
The chickens drive him nuts, and I know hes gonna try to get them once they hit the outdoor run.
I use a shock collar on my JRT, because there is no other way.
I use the lowest setting, and he already knows that once the collar is on, he needs to behave . . .
It didnt take him long to figure out going near certain things would cause him to get shocked, and if the collar is on hes on his best behavior- rarely do I need to hit the button these days.
But if the collar is off, watch out small furry animals.
My mutt(1/2 lab and 1/2 cattledog) is not so simple . . .
She is a sweetheart and I dont think she would purposely hurt the chickens, but I can see her struggling with her instincts . . .
She cant decide whether she wants to herd the chickens, or bring them back to me.
But, I am not really worried about her just attacking them, she knows they are mine.
But I do worry that she would be too rough with the young chicks that I have right now.
She has no finesse.
I go to great lengths to keep the dogs away from the chickens, my coops/runs are strong and well-built using multiple layers of heavy screens.
It really comes down to good basic obedience training. I have a chocolate Lab that has been trained for upland bird hunting. She points, holds, retrieves, takes hand signals and voice commands. I hunt over her with a shock collar, but rarely use it. It is there in the rare instance she short circuits and needs a little reinforcement (always on a low setting). When she was introduced to the chickens she wanted to do what came natural. I only told her no twice, and she has left them alone since. They really do only want to please the "alpha", just make sure that is you.
Most often the best idea is complete separation. I have two golden retreivers. Our female has such a strong prey drive, I could never trust her with any of them. The male loves the birdies and tries to make friends with them. He hangs his head over the stock tank brooder we have, and watches them lovingly, while wagging his tail. I have a picture of him lying on the ground outside our chicken run. He is lying on his side, with his face right up against the fencing, with the birds on the other side. He appears to be trying to socialize with them. That said, I still would not trust him around them, as he is a big guy, and might love them to death without meaning any real harm. It just depends on how many birds you are willing to lose.
I think a lot has to do with you being the alpa and teaching your dog what's allowed and what's not. But just as big, in my opinion, is the breed and temperament of the dog. There are some breeds where I truly think you would have to be a master trainer to be able to control their instincts that much (and even then, will they "obey" when you're not around?). As others have suggested, the only tried and true method despite breed and training is keeping them apart. Time is a big factor too, of course, also determined by how much you put into training, as well as breed (natural instincts). Almost six months after getting chickens, I 99.9% trust my male dog around my chickens unsupervised....my female probably 90% at this point. Like chickenwhisperer's mixed breed, I don't think she would intentionally kill them, but she has (a few times) hopped toward them playfully (like when she wrestles with my cats). Maybe time will cure that (she's a younger dog).
Since your dog has already killed before, you're going to be in for a rough road with training. Definitely keep them separated until you determine whether you're able to change your dogs mindset or not. A shock collar may be your fastest option for training, although I'm not a big fan of them personally.