Serving
The serve cannot be aimed, but you can vary the speed to make it more difficult for your opponent to return. To hit a power serve, time your swing so that you are striking down right when the tossed ball is at its highest point. The serve will travel much faster and shows a trail behind it to indicate that it is a power serve. Once you have gotten the opponent off balance by serving hard, change it up and hit a high, slow serve. Wait to swing until the toss drops low, almost back down to your player's eye level.
Hitting with Spin
The direction in which you swing the Wii controller determines the type of spin that is put on the ball. To hit with topspin, go from low to high while swinging forward. Go from high to low while swinging forward to put backspin on the ball. This makes the ball skid off the court when it bounces to throw off your opponent's timing. Lob when your opponent is at the net by lifting the controller straight up without moving it forward. This is also a good tactic in singles when you are pulled wide off the court and need extra time to recover.
Strategy
Try to play long points early in the game to make the opponents start sweating. This slows them down and makes it easier for you to pick up some quick points by hitting winners later in the game. The bots recover their energy after each game, so there is no cumulative effect toward the end of the match. Unforced errors are rare in Wii Tennis, so you will have to attack the opponent in order to get a chance to end the point. In doubles, use topspin to hit an angled shot to pull the backcourt opponent off the court, then step in with your net player to hit the ball into the open area. When playing a CPU-controlled opponent, try hitting the ball right up the middle at his body. This can confuse the bot and cause it to make an error when it decides too late which shot to hit.