This game is relatively easy after a bit of practice but to help you along the way, here are a few techniques.
1. Move back away from the ball and jump late. This will create a flat trajectory and ball will just go over the net causing problems for your opponent.
2. Get the ball near to the back wall and then jump, making sure that the ball bounces off the back of your head. This cause the ball to rebound off the back wall and over then net. A useful move in later levels.
3. Hit the ball right near your opponents back wall. In early levels the opponent will likely jump and miss. In later levels this is rarer but still happens.