You see it. You want it. You buy it. That’s what e-commerce sites are banking on this holiday shopping season.

Having blocked and tackled basics such as shopping carts and credit card approvals, online retailers know a significant chunk of their sales this year will be in the up-sell and cross-sell.

Booking impulse buys, in other words. But striking a shopper’s fancy is harder to do electronically than in the physical world of sounds, smells and ambiance.

A lot rides on successful promotion: Online shopping is expected to top $33 billion this year, up 21 percent from $27 billion in 2006, according to a study from Forrester Research and Shop.org, a professional group for online retailers. Eleven percent of the 2,521 U.S. consumers surveyed said they would do at least 75 percent of their holiday shopping online.

E-mail marketing, as well as on-screen suggestions, pop-up boxes and live chat, are the most common ways of sparking shoppers to buy more than they anticipated. But not all retailers handle these tools well.

Shipping promotions are always a grabber. Sixty-one percent of those polled by Forrester said they are more likely to shop with an online retailer that offers free shipping. And just 26 percent said they would pay for faster, more expensive delivery this year, a significant drop from the 45 percent who said they would last year, the survey found.

Shoppers expect free shipping this time of year; for retailers, it’s a mistake not to offer it, Forrester says.

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Another too frequent mistake is making too many product offers as soon as shoppers land on the site, says Jared Spool, CEO of User Interface Engineering, an online consulting firm in Bradford, Mass.

Shopping for a turtleneck? LandsEnd.com wants you to consider a cardigan sweater, too.

Macys.com does this. Search for “men’s jeans” and 422 items are found, dozens and dozens of which are not men’s jeans. “You have to work really hard to get to products you’re interested in. They’re always thrusting things at you,” Spool says. “It’s department-store thinking,” he says. “They think of their website as a Sunday flier.”

Instead, e-commerce sites should offer suggestions after a shopper has placed items in the cart. That’s a “seducible moment”—after the shopper has found what she wants and before she’s checked out, he says. Then suggest a sweater for the skirt, a picture frame along with the locket. Landsend.com does this with pop-ups, while Anntaylorloft.com runs clickable images down the right-hand side of the screen. Either way works, he says.