Having worked as a nature instructor at numerous summer camps, I've had more than my fair share of run ins with Ms. Ivy. It doesn't like the shade, so I never ran into it much in the Blue Ridge. However, once college drew me down to the flatlands of North Carolina, I couldn't get away from the stuff. It loves the sunny places at the edge of a forest, like a tobacco field or construction site.
To make a long story short, I contracted the rash over and over until it got infected and I contracted gangrene. So, I talked to the a Botany professor at NC State, and he told me about the magic bullet for Poison Ivy, Tecnu. Made by Teclabs, this stuff is like magic. It chemically breaks up the alkaloid urushiol oil, rendering it harmless. Just wash your exposed areas with in 8 hours of contact and you will not get the rash!. Regular soap and water will do the same thing, but you've only got about 30 minutes to get the job done. I've found technu to be effective up to 12 hours after a serious exposure (falling in a bank of PI plants). It really works like black magic. Even if you do develop the rash, start treatment with this stuff ASAP and it really takes the bite out, minimizing your antigenic exposure. However, just try being proactive - avoiding contact by visually recognizing the stuff, and then washing down with Tecnu afterwards.
Tec Labs also makes this stuff you put on beforehand, like sunscreen. I've never tried it, but I would guess that if it's anything like Tecnu, it's a miracle in a white plastic bottle. Sorry about what must sound like a fanatical plug, but take it from someone who poison ivy has nearly killed - it works!