It's a tough world out there, so knowing a thing or two about self defense can mean the difference between life and death (or serious injury) when you're faced with the threat of bodily harm.
Back in 1950 boxing great Jack Dempsey, aka The Manassa Mauler, published a self defense book called Championship Fighting: Explosive Punching and Aggressive Defense (PDF) in order to share some of his knowledge with regular folks.
Jack packs some really solid self defense tips into this well written book, such as- be aware of your environment and potential escape routes, stay away from crowds, master the power of the lurch, watch your opponent's wrists, and never swing, punch straight.
He also shared this knockout technique for delivering what he called a "sneaker punch":
(1) Keep your head in close to the left side of your opponent's head, with your chin slightly over his shoulder.
(2) Maneuver with your left hand until you can grab the inside crook of his right elbow, and thus hold his right arm so firmly that he can't punch with it.
(3) Get his left arm under your right arm, and clamp your right hand under his arm—just above the elbow—just below the biceps. When you hold him in that fashion, he can't hit you; but you are in perfect position to break away sharply and deliver a stunning overhanded "sneaker" hook.
Read 10 Self-Defense Tips From Boxer Jack Dempsey, Circa 1950 here