eyecager:

Fabric Folds tutorial by Erik Gist who has an amazing blog and teaches at Watts Atelier.

Also if it helps, fabric usually is in either the state of stretching or compressing. Helped me wrap my head around things easier.

Also a good thing to notice is that folds in clothing will wrap around the form of the figure, so all those lines will help define the form underneath.  Took me awhile to catch on to that.