Big-box stores like Costco can be big-time budget busters. Walking into a warehouse of merchandise and wandering around with an oversize shopping cart can lead to spending hundreds of dollars more than I intended when I walk out again. I've developed my own 5 rules for maximizing savings when I shop "Big -Box."
1. Have a plan and a list.
I've already checked the pantry and know what's running low and what we need. Stay with the list. The list is the lifeboat.
2. Maximize the trip.
Because I travel across town to shop Big-Box, I also fill-up the car at the Big-Box gas station which typically charges 10-20 cents per gallon less than my local gas station. I also plan to visit any other specialty stores (the Battery Store, the Dollar store, etc.) in the area on the same trip.
3. Make a mental map of my route through the store and do not deviate from it.
This rule is to avoid the wandering that leads to buying things I didn't even know I needed: DVDs, fleece vests, small household appliances (or big household items), seasonal specials (these are usually at the front---run, do not walk, past them) or two-gallon barrels of cheese-flavored popcorn.
4. Buy in bulk when the unit price is less and stockpile.
This is the big rule for getting actual savings on items rather than just a bigger box: larger sizes do not always mean lower prices.
Some items I generally find worthwhile to buy based on unit price:
Generally not worthwhile compared to weekly sales:
Meat--check the weekly sales flyer for the grocery stores instead
Shampoo and personal care products
Convenience foods (see Rule #5 below)
5. Do not buy convenience foods. Not in a Big-Box or anywhere else, unless you're traveling.
Convenience foods are much more expensive than making the same thing yourself. (OK, I buy tortilla chips and Wheat Thins. I don't want to make them myself.) You don't need a case of small box mixes of seasoned rice for a side dishes. You don't. Buy the 25-lb sack of rice and cook it with chicken stock and seasonings. You don't need a case of cookies, buy the flour, butter and chips instead and make a big batch of cookie dough. Freeze it. When you buy Noodle-Helper, you are trading money for time and a colorful package. Slice it, season it, mix it, and cook it yourself.
There's lots of info on the web about clipping coupons, shopping sales and frugal shopping and meal preparation. These are my own guidelines for getting myself in and out of the Big-Box without blowing the budget. Bankrate has a supermarket shopping quiz if you're interested in testing yourself.