1) Stick to your list: Don't look around. Instead, look straight ahead.
2) Stay to the left: Everything to the right is their new items and items they want you to buy. If you go to the left, you won't get as distracted.
3) Avoid free samples: If you indulge in something like chocolate, your inhibitions have lowered, so the next store you go into you're likely to indulge more.