Incentivizing subscribers with discounts and free shipping to spur sales is core to ecommerce newsletters. What's less common is using incentives to drive other email and online behaviors. Here are some of the behaviors -- from the most common to the more exotic -- that the largest online retailers have been trying to encourage with discounts, sweepstakes and other carrots.

Product reviews. Eighty percent of buyers say that product reviews are very helpful when making buying decisions, according to JupiterResearch. Retailers are responding to that fact by not only introducing product reviews and alerting their subscribers, as Office Depot recently did, they're also enticing subscribers to write reviews. Gifts card sweepstakes are the most popular incentives by far. In a March 4 email, Bass Pro Shops offered up the chance to win a $100 gift card for reviewing a product. Alibris, Blair, Dell, Orvis, RitzCamera and Sears have also offered sweepstakes entries for reviews.

The most interesting twist that I've seen on product review incentives is from Northern Tool, which in a Jan. 21 email offered double entries in its impact wrench kit sweepstakes for those that were the first to review a product. There's clearly more value in that first review of a product -- so providing an additional or special incentive for that is worth considering.

Gift card usage. During January, retailers frequently reminded subscribers to redeem the gift cards they received during the holidays. However, a handful of retailers took the additional step of incentivizing subscribers to redeem their gift cards so the retailers could book the value. Bass Pro Shops, in a Dec. 25 email, offered a $4.95 shipping rate when subscribers used their gift cards, while Foot Locker, in a Dec. 26 email, offered 10% off or free shipping. The Sports Authority also offered 10% off.

Friend referrals. Very few retailers use refer-a-friend (RAF) programs. When I did the 2007 Refer a Friend Benchmark Study, only 3% of the retailers tracked via RetailEmailBlogspot used RAF. One of them, Blue Nile, incentivizes both the referrer and the friend by giving them discounts on products. While RAF is rare, lead-generation contests are fairly common and some of those incentivize entrants to notify their friends about the contests.

Gift reminder sign-ups.  About 10% of major online retailers have gift reminder email programs, including 1-800-Flowers, Barnes & Noble, Hallmark, Lillian Vernon, Omaha Steaks, Overstock, ProFlowers, RedEnvelope, Sharper Image and Sportsman's Guide. A few of them have offered incentives for sign-ups, including Lillian Vernon, which in a Jan. 9 New Year's-themed email offered $5 off and promised that you wouldn't forget people when doing your holiday shopping ever again.

Wish-list building. Wish-list creation is a strong signal that a customer is ready or likely to buy. Looking to tap that indicator, Old Navy held a sweepstakes during the holidays, giving folks a chance to win everything on their wish list. They notified subscribers of the sweepstakes in a Nov. 19 email. What's great about this tactic is that it encourages consumers to browse your products, which isn't always the easiest thing to do online. So at its heart, it's a promotion that encourages assortment familiarity and discovery.

Overstock announced a similar program in a Feb. 6 Valentine's Day-themed email sent on its behalf to 1-800-Flowers subscribers. However, Overstock's twist was that the winner of the contest was the highest scorer in its Game-O-Love online game, adding a competitive, viral angle that Old Navy's program didn't have. (Unfortunately, the game itself was pretty lousy.)

Clicks. In the online advertising world you can pay-per-click, so why not in the email world -- although in this case we're talking about paying subscribers. Neiman Marcus essentially did just that in a Christmas email campaign that began with a Nov. 13 email. During that campaign, Neiman Marcus gave subscribers an entry into a sweepstakes just for clicking to look at its Big 100 Gifts List. Like the wish-list creation promotion, it's an incentive to explore the retailer's products, although there's not the same psychological commitment involved with adding an item to a wish list.

What other email and online behaviors could be incentivized? What about rewarding forwards? At the Email Evolution Conference last month, Lawrence DiCapua, Pepsi's senior marketing manager, said that the company identifies brand evangelists like email forwarders and bloggers and promote them to a VIP program within the Pepsi Extras loyalty program. How about offering a discount to subscribers for updating their preferences -- heck, even for just looking at their preferences?

Have you tried incentivizing any of these behaviors or others? Please share your comments.