SEM Keywords Try 'Em All
When buying keywords for your pay-per-click search engine marketing campaigns, buy a lot of breadth and constantly experiment with ways to improve conversion rates. If a particular keyword isn't working, change the ad copy or the landing page. If the keyword still won't convert after you've experimented as much as you can, then pull it from your campaign and test it again at a later date.
—Brian Elliot, COO, Alibris