For Prevention: Bathe with warm water and soap within an hour after contact.(I wish that was only true of my soap but truthfully, any lye soap will work)

For Treatment: Mash a small amount of Scottish Lye Soap with warm water, or use Liquid Lye Soap. Apply to affected areas with a cotton swab or soft rag. Dispose of swab after use.
Do Not Put Contaminated Swab Back On Soap. The oil from the poison ivy or poison oak will contaminate the soap and spread the dreaded itching.

Do Not Wash! Leave paste or liquid soap to dry on skin. I have had several people say that they have simply washed with it for mild Poison Ivy and it worked great.

Reapply as needed after gently rinsing with cool cider vinegar.TIP: Freeze apple cider vinegar and use it when you just HAVE to scratch. The iced vinegar will soothe inflamed skin, will not tear the skin and help keep from spreading. When you use your fingernails it makes your poison ivy worse. Use the ice vinegar, you will be glad. Especially good for children. You smell like a pickle but who cares.