RULE #1:
Learn to ID Poison Ivy's many forms: ground cover, bush & vine.
Not knowing one is exposing themself to PI is when big trouble can begin since no action will be taken.
If knowingly exposed to PI, then it's imperative to wash up ASAP. I use Fels Naptha which is a petroleum based bar soap found in the laundry detergent section at most supermarkets. This is a somewhat harsh soap, so it has a drying effect on skin.
I do a few string trimmer jobs near creeks where I know there's a distinct chance for exposure and will schedule these for the absolute end of my workday. Then make a beeline home to wash up.
If string trimming foliage w/ PI in it, always work L>R to minimize spraying it onto your person. This direction applies to straight-shaft trimmers.
Even dead PI plants/leaves/vines contain the urushiol that can remain 'active' for years. So, be aware of your surroundings IFF you can.