Here is what a very quick bit of post-work looks like if it is any help. At least how I do it. This was just a minute or so using my laptop touch pad so not as good as with my wacom tablet. I created a new layer set to "overlay" mode and did a paintbucket of 50% gray. I then just drew into this with a soft brush set to 50% opacity flipping between black and white using the "X" keyboard shortcut. Then I did a gausian blur on the layer. The right is the starting image, the middle is the post-worked image, and the right is the postwork overlay layer isolated for visibility. I did not pay much attention to the rationality of my folds but you can study folds, photos of actual folds and texts for traditional artists on creating folds to develop a more acurate understanding of fold dynamics then represented in this image. This really did take just a minute though. Still looks toonish but in part that is the lighting and the shaders. You need to add at least noice displacement and IBL/OA to the render ...I usually then use curve effects and photo grain filters in the post work. Hopes this helps a /bit/ :)