Advertisers using search retargeting can see dramatically higher conversion rates and lower cost per acquisition but campaigns need to be set up in a way that essentially protects them from bidding poorly.
Fortunately, most networks offer up some way to control what ads are shown, how often, and of course, to whom.
Bing, for example, just introduced exclusion lists for its retargeting ads.
Bing Ads' search retargeting will let advertisers create audience lists of users who have visited their sites after clicking on search ads. This will provide advertisers a way to use those lists to keep previous visitors (those that meet a certain criteria) from seeing their search ads (for example, advertisers will be able to prevent users who have made a purchase from seeing ads promoting a price reduction).
The exclusion capability is reportedly rolling out to advertisers globally.