You can add a style sheet that only screen readers can read using this snippet of code:
<link rel="stylesheet" type="text/css" media="speech" href="path/to/file.css">
The styles you place in that style sheet should only be read by screen readers, however I'm not aware of any screen readers that support Web standards properly in this area. The best thing you can do is use table-free layouts, and semantic HTML. Mark up headings as headings, lists as lists, body text as paragraph text, etc. And make sure the headings in the document describe a natural hierarchy of information. Do not choose an H3 tag over an H1 tag because an H3 tag displays the font size you want. The markup is irrelevant for style. Choose the HTML tags that best describe the data on your page, then use CSS to style it. If you need more tags purely for styling, use DIV tags to encase block elements, and SPAN tags to encase inline elements, then assign class names or Ids to the DIVs and SPANs.
More on CSS Media Types: http://www.w3.org/TR/CSS21/media.html#media-types