I've always suggested a knife on the opposite hip of your gun. The Kabar TDI is a simple knife for most without much training. Cutting down on their arm while holding your gun in place is the general idea. I teach a reverse grip, edge in method designed to hook the opponents arm away while cutting into the arm, again with a downward movement.
If that idea doesn't sit well, what Mleake said makes good sense. I've always been an advocate of pushing their arm down, while holding your gun down. It's harder for them to raise an arm up than for you to push down. I always add to that a sinking motion, lowering the center of gravity helps if the suddenly try to take you down. Being ahead of this is a good idea. Also, always pivot your gun side AWAY from them, it helps straighten the arm, making it weaker. That's about all I can explain right now, hope it helps.
P.S. my firearm training is no where near as impressive as many of the previous posters, but my martial arts training makes up for it. Kinda.