Watch the screen at the register for errors.

 Sometimes the computers have numbers that are not up to date. Other times you may have selected a product based on the shelf label that ended yesterday or starts tomorrow. It's not about being nasty, it's about being right. If you picked up 10 of something because of a good price, it better be the good price on the shelf. Carrying a shopping list can let you jot down all prices as you pick up items. This can be very tedious but beneficial. Look for a Smartphone app to make totaling and comparing your bill easier!