The practice is, in fact, not in any way common in Japan and generally associated with prostitution and the yakuza. Western media reporting of the phenomenon severely misrepresents this to the point that it is perceived in Japan as a European fad.
Before becoming a living sushi plate, the woman is trained to lie down for hours without moving. She must also be able to withstand the prolonged exposure to the cold food. Her body hair, including pubic hair, would also be shaved.
Before service, the woman would take a bath using a special fragrance-free soap and then finish off with a splash of cold water to cool the body down somewhat for the sushi.
In some countries, in order to comply with sanitation laws, there must be a layer of plastic or other material between the sushi and the body of the woman.