There's a special bodega shopping while wearing backpack technique.

1. Get in a big fight with the cashier because they want to keep your bag behind the counter while you shop.
2. Never turn mid-aisle. Use your peripheral vision to spot your prey, then fling arms out to either side to seize your Milk Duds, Snapple and Twinkies.

Or you could hold your bag in front of you, but then shopkeeper's gonna be even more convinced your robbing them blind.