When you play defense you want to make sure that at all times you are sliding your feet. If you are not sliding your feet and are just reaching to pick the ball the you are not playing good defense.
If you are looking for the ball or want to swipe at it, swipe up not down. If you swipe down then you are most likely to foul him and are taking a gamble if you do not get the ball.
The next rule is to know who you are guarding. If you know who you are guarding you should be able to know what makes him comfortable and what makes him uncomfortable. Forcing him to do what he doesn’t like to do makes it harder for him to score.
You want to make sure that even if you are not able to block his shot, you still do try to contest everything that he tries to put up. You never know he might actually miss because of the fact that you scared him or put your hands up to contest him.
Just make sure that at almost all times you have your eyes on the ball and you eyes on his waist as well. These defending commandments can show how good you are as a scorer and defender to many coaches who may be scouting.