The universal PI preventer here in Canada is Sunlight Laundry Soap, aka 'yellow laundry soap'. It takes away all oils and I have never seen or heard that it damages any known fabric. Not sure if this product is available in the USA, but I mention it for the benefit of fellow Canadians who can't get the other products previously listed. When you think you've been exposed to poison ivy or its cousins, it's the best known product to use on yourself, or your dog or kilt etc. Of course ordinary soap probably works just as well, but the yellow stuff is reassuringly powerful.