The company: Woot, a $164 million online retailer based in Carrollton, Texas. The company operates six online shops that sell deeply discounted products -- say, a cheap GPS device or a case of wine. Each Woot shop sells only one product at a time, for one day only.

The idea: One of the first retailers on Twitter, Woot uses the service to tell its customers what is on sale that day. Tweets go out at midnight to the company's 1.5 million followers. "We used to say that there's a ton of people who'd visit Woot every day if they thought about it," says Dave Rutledge, who helped his brother Matt start Woot in 2004 and who now runs its creative efforts. "Twitter can help trigger that." Once Woot has sold 90 percent of its inventory for a particular product, which can often happen in just a matter of hours, Rutledge sends a follow-up tweet to alert potential buyers who may be sitting on the fence that time is running out. "That shows people that the item has been approved by the community," he says.

The result: Tens of thousands of people click on Woot's Twitter links every day, and during the company's rapid-fire sales, or Woot Offs, Rutledge estimates the number is in the hundreds of thousands. Conversion rates -- that is, the percentage of users who go on to buy something -- are substantially better than the site's average. Since Rutledge introduced the Twitter stream in January 2007, annual revenue has more than doubled.

How to get retweeted, Part II: Keep messages short. Twitter limits messages to 140 characters, but if you want to get people to tell their friends, it helps if you give them room to add a brief comment of their own. One recent example: "RT @woot: $19.99 : Samsung Bluetooth Headset WITH $20 mail in REBATE!!!!! < pretty awesome, just pay 5 bucks shipping!!" For Rutledge, the magic number to ensure lots of retweets is 110 characters or fewer.