You can do hundreds of sketches, but it'll never prepare you for painting on a supermodel. There's no rehearsing, nothing faked, and you only get one shot (except for the unfortunate subsequent Photoshop tweaking). That being said, I did practice. I did hundreds of sketches; I rehearsed on four different models, including a lingerie model and my very patient wife. I also spent hundreds of dollars on makeup and body paint, and became an expert on the subject. But even this did not make the act any less nerve-racking or my hand any less jittery. The model, Bar Refaeli, was a consummate pro. She ignored me completely.
I have the greatest job in the world. Sometimes.
Photo credit: James White