You need to turn off caching. To do it for your session only, go to Administration, Advanced Tools, Design Tools, Cache Tool, and click the Disable CSS Caching button.
You can also turn off caching for everyone. See the skinning overview document for more information.
When you're done working, you can copy your skin directory to a new name, then choose that new skin as the site skin. This will force a cache refresh for all visitors.