Skirts and dresses are notorious for being difficult to rig.  One of the classic tricks is to group everything below the abdomen as the hip and to add body handles to help posing. 
With body handles you can pose the "thighs" in the dress back to an unbent position and then bend the dress into a sitting position with a body handle.
PhilC has some tutorials on added body handles, though I don't know how good they are as they use the setup room which I never use.  He also has tutorials on rigging and joint parameters.
http://www.philc.net/tutorialsIndex.php

Chris Cox