I have the unique responsibility in my office to be in charge of the Costco run. My office is fairly generous because it supplies things like butter, cream cheese, brownies, and other items that help employees prepare lunch. I try to plan my trips and limit my purchases. Living on my own, its not easy to stockpile, but with some careful planning, I can take advantage of deals I find.
I buy prepackaged chicken breasts to keep in the freezer. It may not be the most economical way to buy chicken, but the convenience of cooking two up and keeping them in the fridge for a week of meals is invaluable.
Prepackaged meals are usually high in fat and calories not to mention more expensive than cooking from scratch, but if the choice of dining out or throwing something in the microwave it may be worth the purchase. I keep a few easy comfort foods in the freezer to prevent unnecessary fast food temptation at bay.
Solid Pack Canned Tuna is a good staple to have around for sandwiches, salads, and casseroles. Solid Pack may be appear to be more expensive, but its better quality and more cost effective if purchased on sale. I try not to spend more than $1.00 per can.
Chips are inexpensive and I can divide the bag out into convenient snack packs using sandwich bags.
Don't fall for the huge packaging. If you aren't going to use the item within a year, it may be a waste of space. One way to help fight this excess is to go shopping with friends, co-workers, or family members and splitting an item. While I don't need 12 cans of broth, I can easily store 6 and split a pack with a co-worker.
Check prices and cost per units. Just because Costco sells it doesn't mean its a bargain. Get familiar with the items you regularly purchase and don't be afraid to research items that look appealing. Impulse buys will kill a budget and many Costco purchases are made because you never know if the item will still be in stock on the next trip. Use the 24 hour rule. If you didn't plan on purchasing the item, don't. Go home and sleep on it.
Finally, take a set amount of cash with you. Don't take checks, credit, or debit cards. Its easy to justify a "few more dollars" if you have that possibility. Remind yourself you don't have to spend the whole amount either.