If you see one, your job is simple, GET BIG. Open up your jacket, put something over your head to make you look bigger. If you have small children, put them on you shoulders. Next job, YELL. Not for help, just make noise that will dicourage the mountain lion. If those fail, get sticks or rocks (somehow without crouching down) and throw them at or near the cat.

DO NOT TURN AROUND OR RUN. This is what the cat wants, it's its signal to go over and slaughter you like a small goat.