Problem: Often hitting hard serves long.
1. For players of average height, hard, flat serves have to just barely clear the net, or they will go long. Only very tall players can get hard, flat serves in consistently enough to make them pay off. Adding some topspin will increase your margin of clearance over the net to several times larger. The most preferred power serve among advanced players has a mix of topspin and slice.
2. Meeting the ball too low is like making yourself shorter, thus reducing the vertical angle from your racquet over the net to your target area. A low contact point also disrupts the upward whipping action you create at full extension. You should meet power serves at full upward extension.
3. You might be meeting the ball too far back. Generally speaking, meeting the ball more in front of you will make you hit lower. Either you're tossing too far back, or you're leaning too far forward before you've swung, thus getting ahead of the ball.
Problem: Often hitting serves into the net.
1. As noted previously, hard, flat serves have a tiny margin of clearance over the net. Hitting some topspin on the serve will allow you to hit over the net by feet instead of by inches.
2. Just as meeting the ball too low can make you hit long, it can also make you hit the net, because it reduces your margin of clearance over the net and disrupts the mechanics of a proper serve.
3. You might be meeting the ball too far in front of yourself. Try tossing less forward or leaning in more just before you swing.
4. You might be fooled by the illusion that you can hit down on a serve and get it over the net. You would have to be well over seven feet tall for this to be physically possible. Most of us have to hit upto get a serve in.
5. If your toss peaks much higher than your point of contact, the ball will develop a significant downward vector as it descends toward your racquet. Try tossing no more than a few inches higher than your point of contact.
6. Make sure to keep your head pointed upward until at least a split second afteryou've hit the ball. If you pull your head down too early, you'll pull your racquet down with it.
Problem: Unreliable second serve.
1. Remember: "Spin goes in." This popular saying really should be "Topspin goes in." Roughly 90% of second serves hit by the pros are kick serves, either topspin or twist, because these serves have a huge margin of clearance over the net. You can hit a good topspin or twist serve four feet over the net, and the spin will make it dive down into the service box. It will then jump up out of the opponent's comfortable hitting zone.
2. Here's another useful saying: "For spin serves, more is more." Contrary to what most players do, you should hit your second serves with at least as much swing speed as your first serves. A faster swing just produces heavier spin, which increases both the likelihood that the serve will go in and the height it bounces above your opponent's comfort zone.
3. If you're trying to hit topspin serves, but they're unreliable, try rotating your grip somewhat counterclockwise. Your grip should be at least Continental for hitting spin, and for heavy topspin you might go as far as an Eastern backhand
Problem: Not enough power.
1. Use a loose, quick motion and full extension. Don't try to muscle the ball. You create power by throwing the energy of your legs, torso, and arm upward, then letting that energy translate into whipping the racquet upward. Keep a loose wrist to allow this whipping to occur, but don't try deliberately to snap your wrist. It will snap naturally if it's loose and you're throwing your energy upward properly to a contact point at full extension.
2. For power, make sure you're meeting the ball somewhat to your right. If you have to lean left to meet the ball, you'll lose much of your momentum.
3. Lean forward into your serve. Your legs should push up and forward as you go to hit the ball.
4. Make sure to turn your back slightly toward the net as you coil your upper body to get ready to hit. The uncoiling as you swing makes an important contribution to your power.
5. Try to use a continuous motion from the start of your backswing to the end of your follow-through. Your backswing adds nothing if you stop at the "backscratch" position before swinging up at the ball.
6. Don't try to actually touch your back in the "backscratch" position. You want your racquet low and the elbow of your hitting arm high, but with a proper motion, you won't be able to actually touch your back.