If a bar of soap is going to be put in someone's mouth, the person who had the soap in the mouth should drink orange juice to lower the pH that the person has consumed. (Thank you to Richard for providing this advice!)
Supposedly pure soaps such as Ivory and Dove are best. (If someone wants to correct me, or add another brand, please do).
Tearless or baby shampoos are preferred. (Organic ones, maybe, but I haven't done the research to make sure).
Barbersol shaving lather supposedly has the least amount of alcohol.