Hitting Long

Assume a right-handed player.

Problem: Often hitting overheads long.

Corrections:

1. As you swing forward for an overhead smash, your string bed goes from facing upward to forward to down. If you're hitting long, you're probably catching the ball late, while the racquet is still facing at least a little too much upward. Meeting the ball earlier, more in front of you, where the racquet is facing forward, should solve the problem.
2. Pointing at the incoming ball with your left hand will help remind you that you want to meet the ball out in front.
3. Don't do a windup. On a serve, you have plenty of time for a windup, but on an overhead, it can make you late. You'll get plenty of power on your overhead without a windup.
4. Learn to scoot backward quickly with your racquet set back, ready to swing.
5. Forcing your wrist to lay back or lock up will keep the racquet from whipping forward properly. Let your wrist snap forward as a natural consequence of your swing. Do not try to snap it forward deliberately. It will snap at the right time on its own, and trying to snap it independently will cause a number of problems.
6. If you get caught in a situation where you can't meet the ball in front of yourself, try hitting it with a mixture of topspin and slice, much like you would the common spin serve. The topspin will help force the ball down into the opponent's court.
7. If a ball gets too far behind you to hit any kind of normal overhead, use the straight-armed "windmill" or "sky hook" overhead, which allows you to maintain a vertical plane in your racquet face even with the racquet well behind you.

Hitting the Net

Problem: Often hitting overheads into the net.

Corrections:

1. You might be meeting the ball too far in front of yourself. Try swinging a tiny bit later.
2. Pointing at the incoming ball with your left hand will help to keep the front of your body from tipping downward too early.
3. Keep your head pointed up at your point of contact throughout the swing. This will help you see the ball longer and avoid pulling downward on it.
4. Do not try intentionally to snap your wrist. Your wrist will snap at the right time if you just keep it loose. By deliberately pivoting at the wrist, you greatly reduce the effective radius of your swing, which makes even the tiniest timing error disastrous. You're also likely to eventually hurt your arm.
5. Make sure you're meeting the ball near your full upward extension. Letting the ball drop too far makes you more likely to hit down on it too much. You generally need to move toward the net more to catch the ball at a higher point.
6. Beware of hitting down. You want to hit smashes mostly forward. Gravity will do a good job of making the ball drop. On a really short overhead, you can hit down a little, but there's usually a lot less room for hitting down than there seems to be.

Hitting Wide, Lacking Power, and Inconsistency

Problem: Often hitting overheads wide.

Corrections:

1. Don't aim too close to the lines. Leave a couple of feet of safety margin.
2. Use either a continental or an eastern forehand grip. It's possible to hit an overhead with a semi-western or western grip, but your racquet will want to be facing at an awkward angle, and you'll have to force it to aim into the court. The continental should be your main serving grip, so you'll be used to it, and it will help when you want to spin your overhead. The eastern forehand grip is not recommended for serves, where spin is extremely important, but it can work quite well on overheads, where you can usually hit flat. With an eastern forehand grip, your palm faces the same direction as your strings, so it's easy to feel in which direction you are aiming.
3. Make sure you're facing mostly sideways when you swing. 

Problem: Lack of power.

Corrections:

1. Keep a loose shoulder, elbow, and wrist. If you stiffen up and try to muscle the ball, you'll get much less power than if you keep everything loose and let your arm and racquet whip up and forward.
2. Transfer your body weight onto your front foot as you meet the ball in front of yourself.

Problem: General inconsistency.

Corrections:

1. Take lots of little adjusting steps as you position yourself to hit the ball. As noted earlier, meeting the ball too far back or in front can cause errors, and positioning is a major factor, along with timing.
2. Don't try to hit the ball with all your might. A quick, relaxed swing will give you more than enough power. If you throw yourself at the ball too violently, you're likely to be off balance, and hitting it excessively hard just magnifies any small errors in timing.