When you are serving, just when the ball is at the highest point quickly flick your wrist. You will see white smoke from the ball and it will be difficult to hit. This may require some practice.
When serving, hold your arm in the air above your head so that the Wiimote is pointing upwards. Hold B and move your arm downwards. Do not move your arm too quickly or slowly. The ball should travel very fast and there will be a white glow coming from it. Note: this requires some practice.