The first concern any coach should have when teaching defensive techniques is basic body position. Only after a player can assume a strong and balanced defensive stance can she progress to other defensive techniques.
The key to good defensive position is balance. A player has to be able to move forward, backward, laterally -- all with quickness and agility. The center of gravity has to be kept low while the player is poised, ready to move in any direction. In the basic defensive position, players should place their feet approximately a shoulder's width apart, with hips low and back fairly straight. Players should be balanced on the balls of their feet for even body weight distribution.
To teach the basic position, have the players assume a relaxed stance with knees bent and with hands resting on the knees. Then have them drop their hands down to their sides, all the while keeping their heads up. Instruct them to move and keep low. They should slide laterally as well as move forward and backward. Make sure they can move in any direction without crossing their feet.
Even while keep low, players have to keep their heads up because they position of the head is important to body balance. Players can't let their heads get too far forward or to one side because they'll lost that balance. A good technique is to have the players look pretty much straight through the player they're guarding. We don't have them pick a spot -- the belt buckle, etc. Players should rely on their peripheral vision to see the whole court situation rather than focusing on the offensive player's navel. After players work on maintaining balance and moving quickly in any direction, they can begin to practice more specific defensive techniques against the offense.
Billie Moore from "Basketball Theory and Practice"