Poison Ivy, and all it's many iterations, freaking suck. I used to get it really bad when I was a kid. If it's already manifested, soaking cheese cloth in strong, dark tea, and putting it on the rash helps the healing along, and can help with itching, too.
If you think you were exposed, it takes time for the urushiol (oil that causes the rash) to bond to the skin and start the rash, so you have a window to get it off. Firstly, throw away the clothes. The oil is very sturdy, and will often just spread to other clothes or the washing machine and prolong your agony. Secondly, turn the shower very cold and hop in. It sucks, but not as much as poison ivy. That will congeal the oil a bit, and close your pores. Thirdly, scrub yourself with alcohol to remove the oil. If you want to get fancy, lathe yourself with big tongue depressors after pouring on the alcohol, but just scrubbing with alcohol should be enough.
It's not foolproof, but it gives you a shot to avoid a crappy couple of weeks. Good luck, and don't scratch. Especially if it's on the twig and berries...