Take a look at the skinning overview document. There are four keys that should be disabled while you are working on skins. Be sure to enable these again when running in production. With these disabled, Firebug will show you the actual file names where the CSS directives are located, and changes you make to those files will be reflected right away.
<add key="CacheCssOnServer" value="false" />
<add key="CacheCssInBrowser" value="false" />
<add key="MinifyCSS" value="false" />
<add key="CombineCSS" value="false" />
Jamie