Every time i try to bend the waist or legs of any of my models, the skin gets "stretched" out and looks very unnatural. I've noticed that several images here at renderosity have overcome this problem. Is there a special editor or program to do this? Or is it as simple as after-rendering touch-ups?

Photoshop or some other similar graphics program is often the answer to your question.

For nude females there is a wonderful model at MorphWorld (link to the right). Unfortunatly there is no corrisponding nude male, nor are there clothes taht will conform to them. For other figures the joint parameters need to be fixed. There are several attempts, each with some degree of success, here on free stuff.