Well, that used to be done via little code tags in your HTML. Over the last few months, Google has rolled out a tool to let you do it across your entire site by previewing your live site graphically, and simply highlighting parts of it.
Those snippets in the results carry a lot of weight; people assume that the sites with snippets are for the more popular and professional businesses.
Make the most of this TODAY. (That’s right, I used CAPS. This is so simple and such a great upgrade to your Google presence!)