I buy groceries once a week at a grocery store, and I shop at Wal-Mart once a month for all of my non-food supplies. I keep a typed list of items I frequently buy at each store taped inside my pantry door. When we run out of an item I highlight it on the list. The night before my monthly Wal-Mart shopping I take my list through the house and highlight what we need. I basically buy the same amount of toilet paper, paper towels, paper plates, toothpaste, etc. each month.
The night before my grocery shopping day I make a weekly supper menu and then highlight the items I will need to prepare each meal. I also know how much milk, cokes, juice, cereal, produce, etc. to buy to last us one week. I rotate several of the same meals for breakfast and lunch so I buy a week's worth of supplies for those meals too. I have one month's worth of suppers typed and taped inside the pantry. I start at the top, cook each meal and when I get to the bottom of the list I just begin again. I do try new recipes on occasion, but the master supper list helps me to make a one week meal plan and grocery list without too much thought. I have tried shopping for 2 weeks and even one month of groceries, but it is just too overwhelming for me. It takes too many carts (3) and I still have to go back to the store weekly for milk and produce.......so I have decided that is just simpler to buy for one full week at a time.
I have the items on my grocery list typed in the order that they appear in the store as I walk through. This is a big time saver.