Shop mostly on the supermarket's outside perimeter

Whenever possible, stick to the outer perimeter of the store. In other words, fill your cart with items from the outer aisles – that's where the fresh produce, fruits and vegetables, the dairy aisle, chicken and lean meats usually are.

"Produce and dairy and other staples often are placed as far from the front door of the store as possible," explains O'Rourke. "That way you have to walk past all the more expensive and probably less nutritious foods before you get to what you wanted." That means if you want to avoid all the tempting junk food, "it's best to stay out of the maze of the center aisles," adds Nestle.

Don't shop hungry

Even with the best intentions, it will be hard not to make impulse purchases when you grocery shop on an empty stomach. "People are much more suggestible in the store environment when they are hungry," says O'Rourke, so eat before you shop, or enjoy a piece of fruit while you peruse the aisles.