The distance from the animal is directly proportional to your height. Do not kick with your knee bent. Swing your entire leg forward like you are drop-kicking a football. This allows greater inertia to be transferred. This page is not responsible for injuries obtained by falls and subsequent scratches by the cat.
Proper footwear is critical.
See notice above. Heavy duty steel-toed work boots are an excellent choice. Soft toed sneakers don't transfer the force as well as the boots but allows the satisfaction of feeling that meaty thunk of purring flesh against the foot. Make sure that there is plenty of distance from the point of contact and any object. A wall if you are in the house, for example.