Ikissmykiss and salty are right on the money. Any noise and lots of it will help tremendously. The desire to do well at the scent of birds is also crucial. The thing you do not want to do most is give the dog any attention for a negative reaction to noise and or gun fire. A dog will learn real quickly to be scared of noise because it is coddled, loved and secured with the negative reaction. When you first fire your gun act as if everything is normal and shooting is what is supposed to happen in the field.

I have an english pointer and a lab that I have trained to work a little harder at the sound of the shot gun. The pointer goes into "bird down" mode after the shot and my lab will hit the water even if I miss when we are duck hunting.

We got our lab from the pound and she was gun shy as h#ll. She would even piss herself if I worked the action. To cure that I left the shotgun out so she could see it and I would pick it up occsionaly and work the action. Every time I did that she would get scared, but I would not even make eye contact with her. Out in the field meanwhile (before season) when she would stumble onto a bird (pheasant ,quaill, etc...), I would make a big deal out of it and give lots of praise. Eventually she didn't care about the shotgun and proceeded to hone her skills of finding birds regardless of what I had in my hands.