Move along a triangle
One of the most dangerous mistakes the average person makes during a fight is to move in straight lines, either forward and backward, or side-to-side. Imagine a vertical dividing line along your body, dividing your body into left and right halves. The aggressor is probably going to attack some point along or around that line: your face, your throat, your heart, or your groin. Your goal is to move that line out of the path of the attack and change the distance of the target from attacker.
The attacker has mentally committed to striking to a particular target. His brain has sent the signal to his fist that the intended target is located at a particular distance in a particular direction. When you change the target's coordinates, it spoils the effectiveness of the attack. The attacker may be able to recover from a change in target location or change in target distance alone, but changing both factors is your best bet. Then, even if it does connect, the strength of the attack will be greatly diminished.
Moving in a straight line backward and forward changes the distance, but does not move your centerline out of the attack path. Moving laterally changes the location of the centerline, but not the distance. Moving along an imaginary triangle changes both. Imagine standing with both feet on the point of a triangle and facing the bad guy. The other two points of the triangle can either be in front of you or behind you. Each of the other triangle points is about one medium-large step away. Step one foot onto either of the two available triangle points. Now you have changed both the distance and target location. Bring your other foot back to the stepping foot, and you are now at the starting point of another triangle.