We occasionally suffer from this awful affliction ourselves. Here are our best tips to avoid the itching and misery. First, familiarize yourself with the enemy so you can constantly watch for it on rides, recognize it when you see it, and avoid it as much as possible. There are good sources of pictures and information on these poison plants on the internet (start your search with "poison ivy"). And, a simple rule to remember is "leaves of three, leave them be."
The insidious thing about these plants is that they look different as the seasons change. Maybe worse, the oil that causes the itching, rash, swelling and oozing, can last for years. Which means, that if the leaves of a poisonous plant brush against a seldom-worn pair of tights, which you then store for a long time, you'll get a fresh dose of itching the next time you put on the tights (if you didn't wash them in between).
It's important to understand that the plants' oils hold the danger. A common myth is that you spread the poison by scratching. But, technically, that's not what usually happens. It seems like you're spreading the oil by scratching because the itchy area grows. But, it's not really spreading. What's happening is that the area that got the heaviest dose of oil reacted first and started itching first. After a while (sometimes even a day or two), areas that received a less-potent amount of oil start itching. So, it appears to be spreading but it's really just a delayed reaction to the original contact.
So, how are you supposed to conquer this fierce foe? Our advice is to avoid it and when you can't, to dress for it by covering up exposed flesh. If you can prevent the oils from getting on your skin, you won't be itching all night for the next month. Just make sure to remember to undress with great care when you get home. Turn things inside out so they can't contaminate you as you strip. And have some one who's not allergic put your clothes in the washer and clean them in hot water.
Anytime you believe you brushed against a poisonous plant, you should wash the areas ASAP with a strong soap such as Fels-Naptha. There are also products available in pharmacies designed for bathing and treating the skin. The goal is to remove any and all plant oils from the skin, which should minimize the rash you develop, if any.
If, after your best efforts to avoid poison plants, you still manage to get a dose, you may be able to allow it to run its course, depending entirely on how bad the dose and how allergic you are. If the rash gets around your face, you should have it checked by a doctor who can prescribe something to calm down your body's reaction to the chemical.