I don't recommend setting CombineCss to false. If you do that the browser will cache it and we have no control over caching in that case.
In newer versions of mojoPortal one should leave those settings alone and after editing css just go to Administration > Advanced Tools > Design Tools > Cache Tool and click the button to reset the skin guid.
For older versions you can use the settings Jamie mentioned except for the CombineCss setting. I do not recommend ever setting that to false.