I always use the Orbisius Child Theme Creator http://wordpress.org/plugins/orbisius-child-theme-creator/ which is free and really good. The child theme is created directly in your installation without the need of downloading and installing anything. Actually, what Donna said would definitely be the best solution. The Responsive Theme (http://wordpress.org/themes/responsive) for example has the Responsive Labzip, which is a pre-made child theme