If you JUST got it, (i.e. it's not itching yet) I always use dish soap and cold water. Even better if you have one of those "Lava" bars for getting grease off.
If you already itch, I'm no help. I always end up getting poison sumac when I'm frolicking in the swamp! A few years ago my boot stuck in the muck, and I fell onto my knee, I must have gotten the oil on my pants at some point, because in a week my knee was, literally, the size of a volleyball. It was a rough summer, as I cut just above my knee with my chainsaw a week later. Just my luck my college ball coach decided to start fall ball early that year. Catching wasn't the most enjoyable thing for the following weeks!