For a reliable bargain on everything from dog food to diamonds, head to a warehouse club, such as Costco or Sam's Club. These clubs limit their margins to between 11% and 14%, compared with 25% to 30% at supermarkets and mass merchants, according to Michael Clayman, editor of Warehouse Club Focus, a trade publication. Annual membership starts at $40 at Sam's Club and $50 at Costco. Both clubs offer upgraded memberships, starting at $100 a year, that give you a 2% rebate on the year's purchases.

But you won't save much money if you get sidetracked by the bling and the flat-screen TVs. Here's how to make the most of a trip to the warehouse (and spend the least).

Do your homework. Don't try to bone up on brands and prices amid the kids, the carts and the chaos. Comparison-shop online before you leave home so that you know which products and features you can't live without before you arrive.

Grab the deals. You can always count on finding a small but decent selection of electronics, fancy food, jewelry and brand-name clothing. But the treasures – say, a Kate Spade bag or Ralph Lauren coat – are here today and gone tomorrow. Snap them up right away or regret it later.

Be prepared to do it yourself. You're on your own when it comes to getting that hot tub off the warehouse floor and into your car (order online and the clubs will deliver). You'll do the installing, too.

Watch those portions. Warehouse clubs offer better prices because they sell in bulk. But it's no bargain if the food goes bad – or if you bulk up eating it. Studies show that "people eat more when there's more in front of them," says Ephraim Leibtag, an economist at the U.S. Department of Agriculture. The only way to save money and your waistline is to resist the temptation to race through an entire tub of rocky road.

Gauge gas prices online. Warehouse-club deals on gasoline are good but not always the best. Before you drive ten miles to Costco – and waste more gas while you spin your wheels waiting in line – check local gas prices at GasBuddy.com.

Reprinted with permission. All Contents © 2010 The Kiplinger Washington Editors. www.kiplinger.com