Step 1 - Prevention: Avoid it if at all possible. Learn to recognize it.
Step 2 - First care: Get it off ASAP. Given time, the usishiol oil will bond with your skin, so wash it off as soon as possible.
Use some kind of oil-removing product - Dawn dishsoap, detergent, Tecnu, mineral spirits, etc.
Wipe it off carefully so you don't rub it in. (Watch the second video in Post #5 by Marshy). Do this a few times, even using different products.
Step 3 - Second care: Deal with the rash/itch. A number of products can help relieve the itch and/or dry out the rash if it's bad enough.
IvyDry, Tecnu Extreme, etc. If it's really bad, the hospital shot does wonders.
Step 4 - Third care: wash your clothing/tools/other infected items carefully. Clothes should be run in the washer by themselves, maybe even multiple times.
(Some suggest running the washer empty after that before going back to regular laundry).
Step 5 - Final care: Suck it up and deal with it, but maybe post a pic or two on Arboristsite
Step 6 - Follow up: Smack yourself in the head to help you remember to do Step 1 next time.