Buy a three-ring binder. Buy one that's small enough to fit in your purse. A spiral notebook will work, too, but it's easier to add or replace pages in a binder.
Create food categories. Dedicate a page or two to such purchases as canned goods, boxed foods, produce, meat, dairy and snacks (you can add nonfood items later). Or download a price-book sheet at organizedhome.com/make-price-book-save-money.
Record your receipts. Make five columns running down each page. List the date, what you bought (including the brand name), the size and the price. (Make a note if the item was on sale.) Jot down the name of the grocery store, warehouse club, food co-op, local farmers market, convenience store or even gas station where you made the purchase.
Bring your binder. You'll be able to look up a product and know at a glance if it's an unbelievable deal. Set maximum price limits for groceries and give them a pass when the cost is too high. You'll begin to notice trends, too, such as how often your store offers sales on canned goods, paper goods, meat, etc.