Shopping at members-only warehouse clubs like Costco is a big money saver -- except when it isn't. In order to really save money, avoid some of the common wholesale shopping mistakes and never assume that Costco always offers the best deals. Make a plan every time you shop. If you play your cards right, you really can save hundreds or even thousands of dollars a year.
Invest in a deep freezer, if you have the space. You can save money buying large packages of fresh or frozen meats, poultry and ice cream at Costco, but you'll have a hard time fitting the large packages in a standard refrigerator/freezer. Avoid frozen convenience foods, unless they're items your family frequently uses -- they add up quickly.
Stock up on paper products and shelf-stable packaged foods such as cereal, olive oil and pasta. Again, you will need a place to store those giant packages of paper towels and toilet paper, but these non-perishable items may be significantly cheaper than buying in smaller quantities. Assess your stock before each trip to avoid over-buying.
Avoid perishables in general. There are exceptions, such as when you're throwing a party or if you're splitting the quantity with a friend, but generally, if it's not going to be eaten right away, isn't shelf-stable or can't be frozen, it's a better bet to purchase smaller quantities at the farmer's market or supermarket.
Comparison shop before your Costco trip. Often, with weekly specials and coupons, certain items will cost less per unit at your local supermarket. You will need to do some of your grocery shopping at the supermarket anyway, so make a list and check it against the store circulars and coupon inserts.
Resist temptation. Stick with your list, because even one or two impulse buys can wreck your whole profit margin. Don't browse the books or electronics unless you have specifically budgeted for these extra items separate from your grocery budget.
Shop at Costco regularly, but rarely -- not more than once a month. More often than that, and you're less likely to go in with a money-saving plan every time.