As a previous cashier, best way to do it is to take your middle finger and swipe down across the center. Like a vertical slap on the bag's face. About the middle of the bag, pull your arm back towards you. It will usually separate the outside wall of the bag from the stack, and when you pull towards you, the bag will "stick" to your finger and then open at the top.

I hated moving slowly as a cashier, cause lines made me mad. So I had to find the quickest way to do everything, and that method works great (for me) until about the last 4 bags.