Some of the trails around here you have to ride through a literal tunnel of poison oak. During the summer months its really bad due to the leaves oozing oil. Ive found that if I apply a decent layer of sunscreen and then just make sure i take a cold shower immediately after riding it will usually prevent any type of reaction to it. I don't know if its the oil in the sunscreen acting as a buffer layer or what but i have yet to get poison oak this year and i am very very allergic. Also ive heard that from some other local riders that they will apply technu before a ride and then shower with it after words.