And it comes as no surprise that retargeting works even better when the creative itself is more tightly relevant to the behaviors a customer evidenced originally on the site. If you visited the camera section of the electronics store, then banners related to that category will obviously perform better than a generic reminder of the retailer's brand.

As always, the problem of devoting multiple, customized creative to a retargeting campaign is scale. It is hard enough to relocate a decent fraction of a single site's audience out there on the other ad networks. But when you start parsing that group into smaller buckets with custom creative, your cost structure rises as your addressable base for each pitch shrinks. Enter dynamic ad creation. If retargeting offers can be generated automatically into creative templates based on that visitor's specific prior behaviors at the retailer site, then you have targeting with a laser scope.

The four-year-old U.K. product content provider MyThings certainly is not the first company to venture into dynamically served retargeting. FetchBack, Dapper and many others do something similar. But this company does have a unique pedigree in having worked directly with retailers for a number of years on their product information, which brings a lot of insight into how content affects people differently at different points in the purchase cycle. The company just landed the retargeting business of Europe's largest online electronics store, PIXmania.com, so now it gets to work with an audience of 25 million monthly uniques drilling into a catalog of 800,000 products. And unlike U.S. firms, MyThings will be managing this retargeting effort across multiple languages and borders.

According to Vice President of Product and Inbound Marketing Anat Amibar, the purpose of highly targeted retargeting is not just to remind a user of the retailer but to match the right content with a person's location in the buy cycle. "It's really aimed at helping them move along the shopping process," she says. "If someone has looked at a specific product or read a review about it, we would come back on the retargeted message with other products with better reviews."

One of the more interesting insights Amibar and MyThings has gained from watching retargeting campaigns is their effect on "return conversion metrics," or the rate at which visitors return to a retailer site and convert at a later date. Every retailer has baseline return conversion rates, and of course a retargeting campaign is designed to improve them. But according to Amibar, surprisingly, "the impact on return conversion actually grows with time within the first two weeks or so and only then tapers off. If you look at the first 48 hours after the initial visit to the site, obviously straight click-through rates on your ads would be higher in the first 48 hours vs. 10 days later. When you compare people who have viewed the retargeting campaigns and returned vs. people who had not viewed the retargeting campaign and returned anyway, the difference in return conversion rates, while still significant, is not as large as it would be after 10 days from initial visits." In other words, the people who have just visited the retailer's site already have that brand in mind just a day or two after the visit, so the retargeting campaign clearly helps in reminding them and promoting them with specific offers.

"However, if I am gone now for a week and I see the retargeting ad, had I not seen this ad there is very little chance I would return and convert. So the impact of seeing this ad vs. not seeing it rose with the first two weeks. That is a very interesting behavior of the numbers that really makes a difference in what type of business model you choose," says Amibar. "If you only work straight on clicks and/or conversions, then you would optimize for the shorter recencies. If you optimize for the full campaign impact, including the post-view impact and return conversion rates, you find that you leave money on the table unless you increase the recency a bit."

Content matters in the custom creative you serve to people at different stages in the purchase cycle. Simply pushing a tightly targeted product offer to a recent visitor is not the best tactic. For instance, Amibar finds that star ratings of other products in the relevant category perform especially well with users who are at a higher part of the funnel. "It is not about coming and buying right now, but did you consider these products?" she says. "It is not so much a direct sales message that gets them back but talking to their state of mind."

If the user is in shopping mode, then help them shop rather than being too eager to pounce. Retargeting is not just a method of "hitting" a user again out there on the network. It is an opportunity to initiate a real conversation that requires a more personal creative approach.

This commentary is insightful. I recommend it to others.