One of those cool CSS3 declarations that you can use today is ::selection, which overrides your browser-level or system-level text highlight color with a color of your choosing. At the time of this writing, only Safari and Firefox are supporting this, and both in slightly different ways. Fortunately, this can be thought of as one of those "forward-enhancement" techniques. It's a nice touch for those using modern browsers, but it just gets ignored in other browsers and it's not a big deal.
Within the selection selector, color and background are the only properties that work. What you can do for some extra flair, is change the selection color for different paragraphs or different sections of the page.