Jewelweed is a natural remedy for poison ivy/oak. It removes the sap and helps the skin dry out. There are several places on the 'net that you can get jewelweed soap but this is the place that I got my soap.
I get poison ivy a few times each year from my cats. Before I found and started using the jewelweed soap, I'd end up having to go to the clinic and get a scriipt for prednisone because the rash would spread and start weeping. This soap stops the rash from spreading even if you don't get around to using it for days. It just takes a little longer to work, you just have to use the soap more often. What we've found that works the best is to lather up the rash with the soap and let the soap sit for a bit from a few minutes or longer if you can. Then rinse and wash again and rinse. Do this every time the itch gets bad and you'll notice the rash will begin to dry up and heal. If your skin becomes too dry and begins to hurt, add some kind of healing oil like olive or coconut to damp skin to help seal in the moisture.
Some one on the home remedies forum posted that Epsom Salt worked for her poison ivy.
I haven't tried that, but I swear by the jewelweed soap. In fact, after I saw how well it worked for me and my daughter, I bought several more bars to have on hand. I gave one bar to my daughter to keep in her bathroom and one to my mom when she got a poison ivy rash. It worked great for her too.