Many times css is cached by server.
Try renaming your css from layout xml and skin folder.
Make sure you are not using any CDN.
Try unmerging css.
If when you clear your cache the stamp on the CSS file stays the same, try turning merge CSS off in Sysytem > Configuration > Advanced > Developer. Clear the cache, then turn it back on to force Magento to recompile your CSS into a new merged file.
Hope this helps.