If out and about and you sense danger then clasp keys in your fist with sharp key protruding through the fingers and jab and slash at eyes and throat.

Alternatively (this was told to me by an army hand to hand combat instructor) bend index finger and push your knuckle hard into the windpipe and keep pushing with all your might, it will bring anyone to their knees and is excruciatingly painful - will give you enough time to make an escape.

I don't go to mythical places with strange men.
-- Douglas Adams, The Long Dark Tea-Time of the Soul.