I have a similar extreme reaction, so this is what I have learned:
Tecnu works pretty well after you have been exposed, but before the rash develops (I realize that is a pretty short window of opportunity). It can also help to stop the oils from spreading. It works by washing the oils off of your skin, and it works better than soap and water, which can sometimes just serve to spread the oils.
Other than that, as a more preventative measure, I would make sure that I cover my legs completely, and avoid touching my pants after I know that they have rubbed against poison ivy.