1. Create a list of weekly purchases. (bread, milk, apples, etc.)
2. Create a list of monthly purchases and another one for bi-monthly purchases. (shampoo, dog food, laundry detergent, etc.)
3. For each item, decide if a certain brand name is important to you or if cost is the major factor.
4. Keep a blank list in a convenient place, such as on the refrigerator. As you run low of a certain thing or if you realize that a particular item is going to be needed, jot it down.
5. Provide the regular list to your errands runner. On a weekly basis, supplement it with the extra list.

After the initial effort, your grocery needs will be fulfilled easily and with minimal work from you.