When serving, quickly flick your wrist just when the ball is at the highest point. You should see white smoke from the ball and it will be difficult to hit. This requires practice.
When serving, hold your arm above your head so that the Wiimote is pointing upwards. Hold B and move your arm downwards without moving your arm too quickly or slowly. The ball should travel quickly and there will be a white glow coming from it. Note: This requires some practice to perfect.