The quickest and easiest way to improve performance of a Google AdWords Group is to find more keywords similar to those that are already successful. Google make this very easy via  either Quick Add or the Keyword Tool.  Simply select the AdGroup that you want to work on. Once inside the AdGroup select the Keywords tab. Once the tab is selected you will see the Quick Add and Keyword Tool links as shown.

Image showing Quick Add and Keyword Tool links

Selecting the Quick Add link will allow you to add words or phrases directly. Having selected Quick Add on the right you will now see a selection of words that Google has chosen based on your currently top performing words – as measured by CTR. Selecting the Keyword Tool link gives access to it. However unlike versions of it accessable in other ways this version has a Radio Button marked Existing keyword. As always you need to review the new keywords proposed carefully as they may not be appropriate. If they are clearly not appropriate you have the opportunity to add them as negative keywords. This will reduce the number of unwanted impressions and will hence lower your cost per click.

Note that if you are a not-for-profit organisation using AdWords via a Google Grant then you do not have access to the Search Network. The Approximate  Avg Search Volume data shown in the Keyword Tool is based on data from both the Google Network AND the Google Search Network. This means the Search Volume numbers shown will be higher than you will be able to achieve.