Create new pages/sections on the site, perhaps even a sub-domain (they seem to keep the same benefits as the root domain when it comes to the search engines). Then, get new links with new keywords pointing to the new area(s).
Your other option would be to get a new domain and do 301 redirects to the new site. Google transfers the PR and rankings to the new domain that way. But, you would then have the issue of different content for the keywords that people were looking for.