I create menu's for clients and off of those menu's I create shopping lists for the week. I also do this for my own family and home. A few reasons for this are; avoiding overspending, sticking to the list you have in hand, helps you to avoid the not-so-healthy options in the store, and avoiding impulse shopping.
Shop at least once a week for your groceries. If you are eating right and avoiding most processed foods, then you will need to shop at least once a week, because the food you will be buying (fruits, vegetables, milk, eggs, etc) have a short shelf life. I personally do one big shopping trip on the weekend, then pick up and restock my produce throughout the week. If you look for deals, and coupon clip you should be able to stay right on budget.