AdWordsAdvisor gives some advice on how to optimize and AdWords campaign.
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In terms of optimizing account performance, and particularly CTR, I would work with this principle in mind: seek to be targeted rather than general.
My experience is that many (most?) people search on specific rather than general keywords (to avoid getting an unmanageable number of results). So if you show them an ad that is highly targeted to their specific search, they'll be much more likely to click.
Not only that, but you're much more likely to a 'pre-qualified' customer clicking. Why? Because they are actively looking for what you've got.