regardless of what you use to make the itching, rash or blisters (myrtle spurge anyone?) ease, one of the most important things to know is that all of these plants secrete an oil as their mode of transporting their misery. What that means is soap or hosing off often just spreads it further around your skin. Your best bet is plain ole alcohol swabs on your skin to lift it off and double wash your clothes that came in contact in hot water (and bleach, if whites).
It's an allergic reaction so benadryl or it's generics that you take internally can provide some relief with their antihistamines. Topically, there are a slew of choices, most only giving some temporary, minor relief.
They do make a cream for pre-contact,that acts like a barrier to skin absorption. I'll have to see if I can find a link for that. Best bet is long pants, shirts and gloves when you know you're going to be around it. (and turn it into an extra service to get rid of it!)