I just realised the summary of some of my pages in Google search results were terrible. So I looked into sites that had good descriptions and discovered Google will use the "description" meta tag for the summary when it's present.
<meta name="description" content="Your Page Summary Here!">
This is easy to do for static pages, and is not the hard to do for dynamic pages. If you use Master Pages its common to define a place holder inside the head tag. This means any dynamic pages can implement a content server control for that place holder and insert a meta tag with relevant information to be displayed in the summary of the Google search results.
You could also give the head tag ID and runat="server" attributes. This means you can find it when your processing the page and dynamically append a meta tag element to its children property.