I carry tools so that if someone bugs me, I can remove the cranks and BB and throw that at them like a boomerang. If that fails, I remove the tires from the rims and toss the wheels so the sharp spinning metal can decapitate them. I'd like to claim credit for that idea, but Goldfinger's henchman, Odd Job, came up with it first.

Sometimes I don't feel like taking my bike apart and putting it back together on the road, so I just be aware of my surroundings. I keep scanning ahead as well as behind (in my mirrors) for potential threats. Curiously enough, this method also protects you from being run over by cars that don't see you. Also, it lets you mentally prepare yourself when you see a vehicle likely to harass you approaching from the rear -- if you know you're about to be buzzed, you don't get surprised and angry when it happens.