Not sure about the efficacy of that method exactly, but for a couple years when I had regular PI exposure due to work, I used a product called Oral Ivy with great success. It's a homeopathic remedy - basically urishiol oil extract taken orally over time to build a tolerance. Think iocane powder. Anyway, it is super effective and I highly recommend it.

When symptoms of exposure have already appeared, I find relief through treatment with hot hot water (as hot as you can stand it). The heat releases histamines, which make you itch like crazy, but after a few minutes your stores of histamines are depleted and it'll buy you 4-6 hours of relief until they replenish.