Dust-Me Selectors (http://www.sitepoint.com/dustmeselectors/) has worked for me along with getting organised in CSSEdit (http://macrabbit.com/cssedit/).

Best approach I've found to keep it all under control and avoid getting in a mess in the first place is to use SASS (http://sass-lang.com/) & Compass (http://wiki.github.com/chriseppstein/compass).