The following technique from CreativeCOW member Todd Groves can be applied to situations where the actor or object that is placed before a greenscreen/bluescreen has some element about them that cannot be separated by the standard approach to keying. The situation that inspired this solution involved a blonde haired actress in front of a greenscreen.
Todd's article is now officially in the CreativeCOW library. Click on the link above if you haven't already seen it in the forum listing. Thanks Todd for putting this together for us.