: It may be convenient when your favorite online merchants e-mail offers for products you were thinking about buying. But wait--how did they know that? Unfortunately, if you've been using Google to browse pages for similar products, or perusing the merchant's Website, it's not a secret. You've been telling them what you like to buy and when--they're just listening.

With the advent of tracking technologies and sophisticated analytics, many Web merchants know exactly who you are and what you are most likely to buy. They know because you tell them through your buying and surfing patterns. This is valuable data, and merchants can (and do) sell it to each other.

: As mentioned above, reputable retailers spell out how they use your information in their privacy policies. Check to see if your favorites sell your information to other merchants. If so, then see if you can opt-out of offers from third parties.

Consider connecting to the Internet through a VPN or private proxy, which will obscure your IP address and therefore your location. This is only effective when you're browsing--once you decide to buy, your identity will be exposed.

Finally, if you're really paranoid, shop at a variety of sites to limit the depth of knowledge about you that any one retailer can acquire. This has its risks, however; buying from that guy on 33rd Street just to avoid shopping too often at Nordstroms is counterproductive.