Poison Ivy/Oak sucks, i got a bunch of it the first week of school all up and down my right arm.
As far as medicines go, benadryll really worked well for me, and putting calamine lotion on the blisters helped keep the itching down.
One thing that no one has said so far, supposedly, swimming in a pool with chlorine actually stops the itching. This seemed to work fairly well for me, and its something no one else has suggested.
about.com <---just go to the medical section and search for poison ivy, they had a ton of stuff last time i checked.
Hope it goes away.