How do you modify text to help your site rank in Google?

Actually they are looking more at related terms as opposed to pure keyword density, they also check density of actual keywords /stems and related terms after the nouns and verbs have been stripped out, they also check the noun and verb density to make sure the text is natural and not just stuffed with keywords.
What they mainly check though is that examples of they keyword exist in the body text, and they are giving more weight to other on page factors like title, header tags, etc.
Also KW density is also still a factor for MSN and yahoo.