Perform a Back Kick

The back kick (Ushiro Geri) is one of the most powerful and useful kicks in the martial arts, and can be adapted to many situations.

Steps

Begin in your basic fighting/guarding stance with your kicking leg back. One common stance is to have your right leg back and a few inches to your right, your body turned almost all of the way to the side and your hands up (right hand by your chin and left 12-16 inches in front of your left shoulder).
Assuming the above right leg back stance, turn your body to your right until you are facing directly away from your target. During this movement you should pivot on your left foot. Your right foot should slide slightly to what is now your right, and it should be further from your target than your left foot.
Look over your right shoulder as you lift your right knee to your chest.
With your ankle bent, drive your right heel straight toward your target using your body weight to add power to your attack.
Finish with your right leg now in front and assume a fighting/guarding stance.

Keep your balance. Some forms of a back-kick can be hard to perform without falling over.
If using this kick during a sparring match, a jump will increase speed, and it will often not be a straight kick as your opponent will be moving.
There are many variations of this kick including adding a step forward to increase power, or jump to increase speed.
If using this kick to break boards or other breaking material, a step forward and straight kick are desired.

As with any type of exercise, be sure that you warm up and stretch before hand and stop if you feel any abnormal discomfort.
Please keep in mind that most back kicks are meant as a lethal attack. If you perform a back kick on a person, try to be careful not to hit their chest region. A blow to the heart can cause severe results. Try not to kick them in the head either. This can cause permanent brain damage. If you are in a REAL fight, do not try to do this often. Use it as either a starter or finishing move. DO NOT perform it on somebody on their knees.
Without the help of an instructor, the technique will be extremely flawed. Do not use this kick in a fight or to attempt to break a board/object unless you have been properly trained. Injury may and perhaps will happen.
Do not to start a fight to attempt this move. Remember: "War not make one strong."
The only way to perfect a back kick is to learn from a qualified instructor.