The theory is pretty straightforward. You need to associate loud noises and hunting "stuff" (gear, shells, guns, vest, decoys, etc.) with things the dog believes are positives.
If you are just starting out in the training and can't make a positive association with a loud noise, DON'T make a loud noise.
Fortunately, most labs are extremely food driven and this greatly aids in initial aspects of training. Preferably, you'll start the noise training very early by making increasingly louder noises while they are chowing down their food (banging their dish around a bit beforehand), making a reasonably loud noise as they are hell bent for leather going after their favorite toys, and give them TONS of encouragement, etc. Try to always have a snack back handy as a reward. You can find a trainer bag at most any pet store that cinches at the top and clips onto your belt.
Dummy launchers are awesome. Cap guns are awesome. My lab can't get enough of retrieving and will literally run himself into the ground if we let him. Don't EVER do that! Stop well before the dog wants to, always. That way, he'll always have a passionate drive to retrieve, thinking it "fun" vs. "work". WRT noise, we simply started the introduction to "gun" noises after basic obedience training by having my wife walk off a distance 50 yards or so and pop the dummy out in front of us. I carried an old beater 870 and would follow the flight path of the "bird", sending the dog after the dummy landed. She would get progressively closer to the point that we could substitute blanks in a 22 single shot over him as she or I threw "birds" from any distance/direction to any distance/direction. 4th of July celebrations are fantastic if you can get the dog in a controlled spot among the comfort of people he knows and just shovel his favorite healthy snacks in his face with every loud boom and give him lots of "atta boy" encouragement. I just use his regular dog food for snacks. Now, any loud noise is cause for sheer euphoria and anticipation of getting to do something fun.
