Update your list. Have a quick snack-never shop on an empty stomach or right after a big meal. Somewhere in between leaves you alert and strong. Listen to relaxing music in the car and take a few deep. That works against a productive shopping trip.
Relaxation really helps.
It is also easier if I know exactly what I am doing-less wandering aimlessly. A price book keeps track of the bargains. It is simply a file of old grocery lists with prices written next to each item. It can be a simple or as complex as you want to make it-the idea is to give you a guide to what the normal price. They see the sale sign and assume they are getting a great deal.