Well, you're doing it wrong.
From the perspective of a right-hander: Hold the bag upright with the notch on the left, use your left hand to grasp the upper-left corner, while using your right thumb and index finger to begin tearing downward. As soon as you break into the actual cavity, begin tearing rightward.
Invert appropriately for a left-hander.
Or just do what I do and keep a pocket knife around. Shrug
