Getting a little extra back on online purchases is as simple as using the right middleman.

Reward portals offer cash, point or mile rebates for shoppers who use the portals to link to and purchase from partner retailers. The sites have taken off in recent years as a way for banks, airlines, hotels and independent businesses to promote loyalty.

The competition is fierce. Microsoft announced last week that its Bing cash-back portal will stop offering rewards on July 30. (Users can continue earning until then, and claim rewards as late as next year.) Microsoft said the experimental program didn t receive the widespread adoption expected from retailers and shoppers. [Bing s] offers were good, so yes, there will be consumers missing it, says Murali Subbarao, the founder and chief executive of Billeo.com, a browser app that compares prices and rewards.

Not using a portal as an intermediary? You re missing out on a valuable part of stacking discounts, says Todd Mera, the founder of award-tracking program AwardWallet.com. Portal rewards accumulate in addition to those earned through a credit or debit card, letting someone spending say, $500 at Dell earn 1% ($5) in credit-card rewards as well as 4% ($20) through Ebates.com.

Used haphazardly, reward portals won t add much to your bottom line. Here s how to make the most of them:

Choose cash

Given the choice of earning cash, miles or points, cash is usually the better choice, says Curtis Arnold, the founder of CardRatings.com, a credit-card comparison site. The value of points or miles fluctuates by reward, which can decrease participants earning power. Cash, on the other hand, is easy to calculate.

Perhaps because of that, cash isn t as common a reward on portals. With Bing s program winding down, FatWallet.com and Ebates.com are two that still offer it. Bank of AmericaAdd It Up also offers cash back when you pay with one of its debit or credit cards.

Shop around

Rebates vary by portal, so look for one that offers the highest rates for the online retailers you frequent. For example, Barnes & Noble fans could get 8% back at Bing, 4% at FatWallet.com, three points per dollar at Citibank ThankYou.com or four miles per dollar through Continental Airlines ShopOnePass.com, among other offers.

Condense earnings...

Participating in multiple programs can dilute earnings if you re not careful, Subbarao says. Consider using a portal that combines rewards with those earned through another program, such as a credit card or frequent-flier account. That helps rack up rewards for redemption faster.

...but stay flexible

Portals and loyalty programs routinely offer bonuses and other deals as enticements, Mera says. Keep a handful of reward portals bookmarked for quick comparison before making a large purchase.

Read the fine print

Before signing up, check site policies governing how rewards accrue. Ebates.com, for example, sends payment every three months, but will roll over rewards to the next period for shoppers who earned less than $5.01. Some programs also offer fewer rewards if shoppers use a coupon code for that purchase.