Concerned about their finances, U.S. residents will spend US$44 billion in retail purchases online during November and December, the period Forrester defines as the holiday shopping season.

The expected growth rate is the slowest since Forrester began tracking e-tail holiday spending, said analyst Sucharita Mulpuru. "The key takeaway from our report is that spending is decelerating," she said.

Twice as many consumers are worried about the economy and about their financial stability this year, so they have reined in retail spending, she said. Forty-five percent of online shoppers plan to spend less than they did last year this holiday season, compared with 20 percent in 2007.

Although the increase in spending will slow, it will still grow in the double digits, so retailers would do well to improve their holiday sales by making sure their e-stores are in tip-top shape. "If a company is going to focus anywhere, they should focus on their Web sites," Mulpuru said.

Specifically, e-tailers should verify that stores don't have broken links or bugs, that information about special offers and product details is available, current and complete, and that the site is easy to navigate, she said.