I just shopped at Costco today! We go almost once a week, rotating the staple items. I work crazy hours and have no time for couponing. I also have brand preferences, which makes cutting coupons difficult. Our usual items - diet pepsi, milk, all paper goods, all cleaning supplies, tide pods, downy, rotisserie chicken, genetic otc meds/bandaids, sunscreen, razors, photos, bread, splenda, batteries, snacks, juices, coffee etc. Another awesome thing to buy really cheap is top of the line spices and the amazing madascar vanilla! Occasionally Costco has saffron! Our local Costco now sells Kerrygold butter! LOVE the king size Kirkland sheets! After 10 years I finally wore out a set! Occasionally they have great deals on gift cards. Last weekend $100 iTunes gift cards were $85.
I am fortunate to have a Wegmans nearby, so I usually prefer to get most of my meats and produce there (can't beat Wegman's produce - smaller quantities and great prices).
The budget killers are the great limited quantity treasures/seasonal items that temp you to spend money. These are also really great deals, but more ohhh I like it rather than I need it items. Don't get me wrong - great purchases but if you are on a tight budget they are quite tempting. Usually items like this I like to think about first but if you leave the store there is an excellent chance it will be gone. Return policy is awesome, though.