Oh yes... those black lines in the first image. I hate those, and I can't get rid of them no matter what I try. In preview, they look like the fabric has snapped or crushed in on itself, and in render they just show up black and ugly.

I've got a skirt simulation going at the moment where the figure is simply sitting down on a chair and at the place where the thighs bend at the hip, those sharp creases are appearing. I try changing the fold resistance and other values, but this just serves to make the rest of the garment mess up (e.g., where before it was draping over her legs nicely, it's now floating several inches above and refusing to drop any closer, and other---stranger---effects such as going right through her thighs even though it started off by colliding correctly with them). And then, when I bring the chair into the picture and make the skirt collide with it, then things start to get really weird.

I can already feel that familiar sense of frustration setting in...