Zanfel™ is a soap mixture of ethoxylate and sodium lauroyl sarcosinate surfactants. When "activated" (worked into a paste that can be spread effectively on the skin), the soap is able to bind urushiol and thus allow it to be removed from the skin by rinsing.
Zanfel™ is unique with respect to poison ivy/oak/sumac remedies in that it is supposed to remove resin from the skin after the rash has appeared. In one study, this effect was present even at 144 hours post exposure. However, it seems logical that at some point post exposure, urushiol is no longer present in the skin and that the allergic contact dermatitis (manifested as redness, itching, swelling, and blisters), would not be lessened by Zanfel™, unless it has some direct anti-inflammatory properties
From the comment section comes this helpful suggestion via White Coat
One of the other things that helps to some degree is "Ivy Block" - it allegedly keeps the urushiol from binding to the skin. http://www.ivyblock.com/ivyblock.phpAlso, check out the post he did “Poison Ivy – Son of an Itch”
Also, TechNu is reported to work as well as Zanfel, but is significantly less expensive. http://www.teclabsinc.com/products.cfm?id=1F5604C8-9D05-4675-56129F6D83DF2417§ion=1