There is also a product called "BELT DRESSING" that is designed for this.
http://autorepair.about.com/od/quick...t_dressing.htm
Also, if the belt has been slipping for a while, get a new one then dress it with the bar of soap or belt dressing.
Wouldn't Ivory or any soap with lanolin cause it to slip more. It might not squeal but it might still slip.
Belt dressing makes it more sticky.