Print stylesheets should be used to reset styles, hide elements, and boost the font-size. It should not be a total re-write of your other stylesheets.
All of your main stylesheets should have media="all" as an attribute. The print stylesheet should have an attribute of media="print" and should be the last stylesheet loaded.
<link href="css/reset.css" rel="stylesheet" media="all"> <link href="css/main.css" rel="stylesheet" media="all"> <link href="css/print.css" rel="stylesheet" media="print">
The print stylesheet should be attached to every page, regardless if it is a print-specific page. File » Print will still trigger the print stylesheet.
With the right media types inplace you can edit the print stylesheet using Firebug and the Web Developer Toolbar (CSS » Display CSS by Media Type » Print)