Proper Defensive Stance
A strong defensive stance is the foundation of post defense. Keep your feet shoulder-width apart with knees bent and your weight on the balls of your feet. Your back should be relatively straight, and your center of gravity should be low. This position allows you to move quickly in any direction and gives you stability when the post player tries to push through you.
Positioning and Body Control
Always try to position yourself between the offensive post player and the basket. This is called being in the "passing lane" or having good defensive position. Use your body as a wall without reaching with your arms. Stay close enough to contest shots and passes, but not so close that you can be easily knocked off balance. If the post player gets the ball, immediately put pressure on them while maintaining your balance.
Hand and Arm Techniques
Keep your hands active and up, extending them to contest shots and passes without reaching. When a post player shoots, raise your hands quickly to block their vision or deflect the ball. Avoid reaching across the post player's body because this usually results in a foul. Instead, keep your hands in their space around the basket area. A good technique is to wave your hands to distract the shooter rather than trying to block the actual ball.
Footwork and Movement
Use quick, short steps to stay in front of your opponent. Slide your feet rather than crossing them over, which can throw you off balance. When the post player tries to move around you, follow them with quick lateral steps. If they drive to the basket, use a low defensive slide to cut off their path. Never let them establish a comfortable spot in the low post where they can easily catch the ball and score.
Avoiding Fouls
Reaching is the biggest mistake defensive centers make in post defense. Instead of reaching, use your body to block space and force the post player to take difficult shots. Don't hold, push, or grab the post player. If they charge into you with the ball, stand your ground with your hands up and let them commit the offensive foul. Physical contact is allowed in basketball, but it must be incidental and not excessive.
Help Defense and Rotation
Post defense is not just about one-on-one play. If your teammate's player cuts to the basket or another offensive player gets open, be ready to help by rotating over. This means temporarily leaving your post player to stop an easy basket, then quickly rotating back. Communication with teammates is important so everyone knows when help defense is coming and who should rotate to cover other players. 
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