Lots of good poison ivy/oak advice here. Having suffered the monster case of poison oak from a Tuolumne trip 20 years ago (which comes back!) I have done some research on poison oak/ivy prevention. The oil (urushiol) is really tenacious. Think of it like axle grease. Any thing that gets off heavy grease/dirt, gets off the oil. AND you have several hours, (not all day) to get it off. Use anything, (and I DO mean anything) that cuts grease after you know you have been in it. Use it like you are getting off serious grime, but avoid harming the skin. All of the things people have mentioned are good, but I have also had success with
Windex, WD-40, alcohol wipes, etc. I take windex on my mtb, really.
I have heard Coleman fuel works.
I get it pretty bad, so if I know I have been in it, I don't rely simply on soap and water. I'd probably even waste some (some) tequila on my skin to cut the oil if I had no choice.
And yes, your gear (your dog, your paddle, your SO) can give you the oil for a LONG TIME after exposure. I have read the oil can last for 7 years on objects.
The prevention stuff that I like is called Ivy Block. Here in Poison Oak land (SoCal) they sell it at the chain drug stores. It has the same stuff in it as Ban Deordorant. The other after-exposure stuff that folks mentioned is Tecnu. Both Tecnu and Ivy Block have good web sites with factual info.
Oh and once you have it (bad), my home remedies (in addition to cortisone, which you NEED once it is mas bad) are ICE and Clorox. Ice it down if its driving you nuts. And if you want the no guts, no glory cure for active rashes....scratch it to death, have a few drinks and douse it with Clorox. Don't skip step 2.