After a ridiculous bout of poison oak last year, someone told me about this stuff called Tecnu. I wish I had known about it sooner! The stuff is amazing. You shower with it after you hike & it takes off any poison oak residue. And it is also great if you already have it, it makes it go away so much quicker than anything else in the drugstore. You can find it at REI and at some drugstores.
I go camping a lot and this stuff is great. A friend introduced it to me and I have yet to get poison oak (knock on wood). We usually put the stuff on our arms 1/2 way through the day and again at the end. I also like it because it does not have a strong odor.