I've always considered myself a savvy shopper, with coupons and calculator in hand. But lately I've begun to wonder. Like when I go to a discount store to buy the cheapest milk but drive away with a cart full of goods I wouldn't have been tempted by at the corner store. Or when I'm sent a coupon via e-mail for a favorite store and find an excuse to browse the sale racks, despite knowing I wasn't planning a trip to the mall.

It's no secret that humans are irrational when it comes to spending, saving and investing. Just ask the behavioral economists or read the studies about how much money we lose, not from stock market declines, but from buying high and selling low.

With the holiday shopping season now officially underway, remember: It's not a bargain if you only bought it because it was on sale. And no deal at the mall is worth taking on debt.

Here are some common mental booby traps that could ensnare you at the store:

Spend more, ship free. Free shipping offers have multiplied along with online sales. But there's almost always a catch. Merchants usually require you to spend a certain dollar amount, sign up for a membership program that may have a monthly fee or use their store-branded credit card.

Mike Allen, founder of Shopping-Bargains.com, said online retailers know what their average sale size is and usually offer free shipping for those who spend a bit beyond that amount. If free shipping is available to people spending $100 and you were planning to spend $90, you may be tempted to spend $10 more to receive that free shipping. But if shipping only costs $5, this tempting offer just tricked you into spending more money.

If you can't bear to pay shipping, mark Dec. 17 on your calendar: 500 retailers are expected to offer no-cost delivery by Christmas Eve. Details at Freeshippingday.com.

Spend more to save. Retailers know that coupons and freebies cause shoppers to spend more money. The offer that really tempts me? When retailers offer consumers who buy $100 in gift cards an additional $25 gift card. I've used this deal to buy a lot of small cards as gifts. I've also taken advantage of this deal for my family when I know for certain that I'll use the gift cards as tender in my daily spending.

But if you are prone to losing things, or if the gift card freebie is so small you'll end up spending far more than the card's value in order to redeem it, skip these offers.

It's on "sale" (wink, wink). The Web makes it easier to price check and comparison shop. But it can still be tough to know whether a sale is truly a sale, or if the original price is "a retail price that nobody even charges anymore," said Barry Boone, CEO of Currentcodes.com. Retailers with store brands typically have more profit margin to play with and the leeway to frequently discount items, Allen said. He said he never shops at some stores without a sale or a coupon "because I know that if I just wait a week or two that item's going to cost less."

I never pay full price at Kohl's or Herberger's, for example. To understand a store's particular sales and pricing strategies, you'll need to monitor prices for items on your list.

Coupon shock and awe. I'm a huge proponent of signing up for merchant e-mails, becoming fans of retailers on Facebook or following them on Twitter. These are the best ways to learn about sales and receive exclusive coupons. Problem is, you'll learn about sales and coupons. Every. Single. Week.

In these uncertain times, I find coupons give me permission to spend money in the name of deals too good to pass up.

I frequently print out these offers and carry them with me, just in case I'm near a store. But it's annoying to print out and lug around so many deals. And many coupons have fine-print exclusions that make them even more frustrating.

Since I know I'm prone to spend more with the offers in hand, why don't I leave them at home? I'm trying, but the irrational deal-seeker in me often prevails.

It's now or never. Yes, inventory is low at many stores. And the ads scream at us about limited quantities and deals that are too good to last. But Boone suggests consumers resist these aggressive sales pitches. "Hold off on the impulse buy," he said, and ask yourself whether you are being goaded into the purchase by the retailer.

So before you spend a penny more, take time to understand your shopping style and your bad habits: Make a list, check it twice. Set a budget. Bring a friend. Leave the credit cards and even the coupons at home. Promise you'll head to the return counter if you overspend. As Boone says: "The only defense a shopper has is discipline."

How do you keep your spending in check? Tell Kara McGuire • 612-673-7293 or kmcguire@startribune.com. Follow her on Twitter: www.twitter.com/kablog.