If you accidentally touch poison ivy and apply jewelweed juice to the affected area before the rash appears, you probably won't get the rash.
The Indians treat already-developed poison ivy rash by rubbing jewelweedís broken stem on the rash until it draws some blood. The rash then dries out, a scab forms, and healing occurs. http://www.wildmanstevebrill.com/Plants.Folder/Jewelweed.html
When exposed to poison ivy, poison oak or stinging nettle, the best thing to do is to immediately take a jewelweed plant, slice the stem open, and rub it directly on the exposed parts of the skin. This prompt action will often be enough to prevent a reaction entirely, and at the very least will offer significant symptom relief.
I also use plantain, which is easier to find, to stop the itch of rashes and bug bites.