I've had good success with using Mugwort for preventing poison oak reactions. When I'm out backpacking and get close to poison oak I always look for some Mugwort near by. I've found it generally grows in the same area as poison oak. I make an infusion, bring a pot of water to a boil and then pour the water over a pot of freshly picked Mugwort and let soak. I also include some Oak leaves if I can find them. After the water cools I pour the infusion over the skin where I think I may have come in contact with the poison oak. This works if you can prepare the infusion within an hour or so of coming in contact of the poison oak. I believe it helps dissolve/wash the oil from the poison oak off the skin so it doesn't cause the skin to become irritated. I've used this approach probably 6-8 times and never got the poison oak rash that I normally get if it comes in contact with me and I do nothing to clean the skin.
