It's easy to zone out at the grocery store register when you're checking out. Whether your weakness is reading the covers of the tabloids in the aisle or scanning the candy shelf for the latest crazy flavor of gum, resist the urge and instead pay attention to the cash register.
Oftentimes, prices ring up incorrectly or sales that were advertised in the paper or marked on the shelf aren't reflected at the register. Keep a close eye as each of your items is scanned, and make sure the low prices you think you're paying are actually what you're being charged. You're certain to catch a few mispriced items every now and then.