I've had it more times than I care to remember. I've found there are tons of creams, lotions, etc. but haven't had much luck with any of them.
Here's what works for me. If I know I'm going to be in or near it, I put IvyBlock on my hands, arms, and face. I wear long sleeves, gloves, etc. As soon as I'm done working in it I change all my clothes and wash with lots of cold water. You need to do this soon, within an hour or so. I've found dish detergent works well because it cuts through the oil. Don't use regular hand soap, this will just spread it around.
If the rash gets bad and you have swelling, try taking Allegra during the day to relieve it. It doesn't make you sleepy like other antihistamines. Allegra is pretty expensive I think, but I had a bunch of free samples the Dr. gave me. I only take Benadryl at night because it makes me too tired.
Here's another trick I found. I've had it so bad I was up all night because the itching was so intense. There's a hot water trick I read about and tried and it worked for me, but the Dr's probably wouldn't recommend it. Take a shower with the hottest water your body can possibly stand. The hot water releases all of the histamines (which is what makes you itch) at once. This will cause the worst itch you've ever experienced for about 1 or 2 minutes. Let me tell you, it's intense and you'll probably want a wash cloth to bite down on. But after this brief intense itching period, you won't itch for several hours until the histamines in your body build up again. It works good at night so you can get some sleep. It worked for me anyway.
Don't use bleach or anything stupid like that. You're just risking an infection.