Make sure you have the following keys set in your user.config and then clear your browser cache.
<add key="CacheCssOnServer" value="false" />
<add key="CacheCssInBrowser" value="false" />
After adding these to your user.config; open the web.config, add a space to the bottom and then save it. This will cause the Application Pool to recycle and load the settings from the user.config.
Once you are finished making skin changes, change these to True.