Many times in the summer when you're outside in the vicinity of Poison Ivy, you will have developed a sheen of sweat on your body. The oil can be transferred to other parts of your body if you rub it and then rub another part. It depends somewhat on your individual tolerance, but some folks have reported that they got it from their pets who had been in it. Also, when you are warm and sweaty, your pores are open and contact will help the oil work its way into your skin. Wash your hands first, then wash the area that has been exposed, then wash your hands again, then go home and take a shower.
Plain water works well.
Cold water seems to work a little better, because it closes your pores fast. Soapy water or alcohol also work. Last summer I was bush-whacking through the woods, on a friends property while wearing short pants and no socks. We hadn't gone very far when I noticed that our chosen route was through a big patch of Poison Ivy. I tried to step carefully, but it was inevitable that I brushed up against it.