In addition to what Aaron said, when you are working with the css, you will want to disable caching by adding the following to your user.config file. After you have finished with the css and everything looks as you want it to, copy your skin to a new folder (myskin-v2), set your site to use the "new" skin, and then set the caching settings back to true.
<add key="CacheCssOnServer" value="false" />
<add key="CacheCssInBrowser" value="false" />
HTH,
Joe D.