A tired child makes that trip to the store quite unpleasant. Bring a little snack so your child isn't hungry and tempted. It might be a good idea if the minute you get to the store, you let your child choose one tasty treat that she can eat while you cruise the aisles. Animal crackers are favorites. Make a game out of shopping.
Children love to be helpful.
Give your child some coupons, and let her watch for those items as you roll down the aisles. Let her choose one kind of cereal, but be specific if there are certain cereals that are off limits. And let her pick one kind of juice. If it's a concentrated juice, make plans to mix it together. If she continues to fuss and beg, calmly stick to your rules.