I've had poison ivy so many times in my life that I think I'm now allergic to it rather than have built up an immunity. Last time I had it, it spread over my entire left arm, part of my torso and some of my left leg.
Best thing out there is Zanfel. Back then you had to order it through a pharmancy, but now I think you can get it over the counter at Walmart. Yes, it's expensive...but it works. I had boils the size of a baseball and after one long application of it while taking a shower the boils were half the size and I had relief for a couple hours.
This in conjunction with a some kind of steriods from the doctor, I think my bad case of poison ivy went away in like 3-4 days. No scaring either.
If someone tells you to try the home remedy of popping the blisters and applying Clorox...run away. Not only does it hurt and may leave scars...it does nothing and really hurts...bad.
Iverest, Ivy Dry, Calamine lotion...they are all anti-itch creams. They don't really do anything for getting the poisonous lotion off your skin which actually forms Poison Ivy.
http://www.zanfel.com/help/rashfaq.html