MILK Costco's Kirkland brand of milk is one of the lowest priced on the market -- plus it is hormone-free!
BREAD I get two loaves of SaraLee bread at Costco instead of one at Jewel for the same amount of money.
KIRKLAND DIAPERS & WIPES I think the Kirkland Brand is an outstanding value with highest quality. It's the EXACT same diaper as the Huggies Supreme (much nicer than the regular Huggies). So you are paying for the "upgraded" diaper at the regular Huggies price. It is definitely worth a try. There certainly are cheaper diapers on the market, but I don't mind paying a bit more to get the significant increase in quality.
ENFAMIL Formula, if you use it. It's the best price I've found. Plus, they used to take my Enfamil promotional checks that Enfamil sends...because they are not coupons. Those checks are legal tender.
KIRKLAND PAPER PRODUCTS Toilet paper, paper towels, tissues and napkins. Very high quality.
KIRKLAND LAUNDRY & CLEANING PRODUCTS Detergent, fabric softener, cleaning wipes, etc. Again, very high quality
Pretty much anything Kirkland brand! You may be able to find really cheap alternatives (if you shop Aldi brand, etc.), but you cannot find anything as high quality for the price.
CARTERS BRAND PAJAMAS FOR KIDS Better pricing than the Carter Stores. Usually *more* than 50% the tag price at the Carters Store (even though you rarely pay tag price at Carters of you have coupons, sales, etc.)
YOUR BIGGEST THREAT Stick to your shopping list -- don't buy extras that you may not end-up using. It can be tempting to throw something "fun" into the cart!
One note for the future: As you start ramping-up your shopping at Costco, they might suggest you pay an upgrade for an "Executive membership." Regular membership = $50 and Executive = $100. However, Executive gives you a 2% rebate per year...and if it does not cover the cost of the extra $50 they will give you a refund of the difference. I mention this because I do so much shopping at Costco, my annual rebate is much more than $100...so it always covers the upgrade AND the basic membership fee.