Watch the register.

In most grocery stores, advertised sales change every week, so the scanners have to be updated constantly. But surprisingly often, they're not, so they charge you the full amount, not the discounted price. To get the deal you're after, keep an eye on the price being registered by the scanner, whether you're operating it yourself or dealing with a cashier. Put all the week's specials in a corner of your cart and line them up on the checkout counter together so you can easily keep track of the prices you're being charged as the items are scanned.

Don't assume that regular-price items are being entered correctly.

After all, the person at the register may not know a pomegranate from a persimmon. On a recent shopping trip, I watched as a cashier entered my three-pound cabbage (39 cents per pound) as Brussels sprouts ($1.89 per pound)-an overcharge of $4.50. If I hadn't corrected him, my family

Collect your payoff if there's a mistake.

Many people don't know it, but big chains like Giant, Safeway, and Kroger will often give you an item for free if the scan shows a higher price than what's advertised. (Tobacco, alcohol, pharmacy items, and dairy are excluded.)

Be persistent.

If your grocer runs out of an advertised special, ask customer service for a rain check-a written promise to sell you the item at the lower price when it's restocked. Make sure the slip specifies the maximum number you can buy for the sale price (usually six). Ask about the time limit for using your rain check (it's usually 15 days).

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Don't be fooled by deals like two for $1.

You don't have to buy two. If the ad says "Buy two for $5" and you want only one, you can still get it for $2.50. If the item doesn't automatically scan that way, point out the sale price to the cashier.

Stock up, but wisely.

Before you stash ten boxes of something on your shelves, check the sell-by date. Even seemingly ageless products like tea bags and toothpaste eventually expire-and you don't want to be stuck owning eight more when they do.

Weigh before you pay.

If you're buying a ten-pound bag of potatoes, put it on the scale before you put it in your cart. Many items weigh less than what the package says.

Don't pay extra for convenience.

It's easier to pick up personal-care products and other nongrocery items at the supermarket. But this stuff often costs more than it does at a big drugstore chain.

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