Straightforward:
* Her skin was as smooth as porcelain.
* Her skin was as soft as a baby's bottom.
More creative:
* Her skin had the velvety texture of a rose petal.
* Her skin was like a cloud, so soft and ethereal.
* Her skin glowed with the warmth of summer sunshine.
Humorous/Slightly Ironic:
* Her skin was so soft, it could make a butter knife jealous.
Remember to choose the cliche that best fits the tone and style of your writing.