One word - Plantain.
A simple garden weed is the best source of itch relief from poison ivy that I've ever found - and believe me I've tried them all.
Plantain is supposed to relieve bug bites, stings, and even the itch from poison ivy and poison oak rash. I was skeptical, but the other night I woke up itching with poison ivy. I have a bug bite balm which I made from several other herbs. I put that on; it helped right away and I went back to sleep.
As the rash matured over the next 24 hours or so, the bug bite balm no longer helped. Finally I plucked a leaf of plantain growing (with abandon) in my yard. I pulverized it between my fingers and rubbed the juice on my rash. Instant relief. And lasting.
I was amazed.
I gathered a nice bunch of leaves and made an olive oil infusion, hoping to make a balm for poison ivy with it. I've found that the infused oil is very good for mosquito bites (immediate and lasting relief), but that it does not work on the poison ivy. Soooo sad.
I guess fresh is best, but I'll try making a glycerite to see if that works on poison ivy.
If you want to try this remedy, remember, it's the lawn weed plantain and not the bannana plantain. Also, don't use plants that may have lawn chemicals (fertilizer, weed killers) on them. The pictures above are of two different varieties of plantain. Both work equally well, but I've found that the narrower-leafed variety is easier to smoosh up with your fingers to get juice out of.
EDIT 7/11: I have found out how to make a poison ivy itch-relief spray using fresh plantain leaves. See this post.