Some pics here should illustrate the general concepts of the wrinkles noted in the previous post. I picked a few figures of 15mm Spanish figures from Battle Honors and AB (white and yellow coats make the patterns more recognizable), and in due fairness, some of the clothing folds are sculpted into the figs. I've posted both infantry and cavalry figures, although the folds are going to be very similar. Most of the folds are not sculpted, which should demonstrate the fact that if one wishes, the folds can be painted in instead.
For really fine work, like the inside of the knees and elbows, I use a size 00 brush with an extremely fine point on it. I carefully drag the brush from the inside of the knee/elbow out, allowing just the very tip to drag and make a sharp "V" to the knee/elbow joint. The rest of the sleeve or pant is painted so that the color is more or less solid in the stressed part of the fabric (the anterior of the leg, posterior of the arm), and the wrinkled part is painted in in a more or less diagonal fold pattern. If anyone is interested, I could do a step by step, but until then, these pictures should show the general idea. Happy painting!