A rash develops not because you got the ivy on you in the first place, but because you fail to wash off the oil that the plant transfers to your skin. If you wash between 2 to 8 hours after exposure, you will avoid the rash.
Most people wash with only soap and water. Dish soap cuts through oil better than soap. But the secret is that you must use friction. A damp soapy wash cloth scrubbed on the skin will remove the poison ivy oil better than any soap alone.
Personally, I have no experience with the rashes, as I am immune to poison ivy.