Many players play table tennis like they were playing pinball. They stand there, and when they have to move, they mostly just reach for the ball. The reason for this is a combination of two factors: they are not ready to move, and they are not in the habit of moving. These are two separate things. 

A coach could stress the proper ready stance and the importance of moving, and try to get the player to be ready to move and to develop the habit of doing so. And it might sink in. But there's an easier way. Put your racket aside. Now imagine you are covering someone in basketball. (Or an infielder in baseball or a goalie in soccer.) Don't think about it; just do it. Then examine what happens. Almost for certain your feet went a little wider, your weight went on the balls of the feet, your knees bent some and pointed outwards slightly, and you leaned forward slightly at the waist. Your knees were suddenly bouncy as you prepared to move in either direction. (You probably raised your arms as well; you can drop them now, but keep the free hand up some for balance.) You are now in a perfect ready position! 

You are now ready to move. Do this regularly, and you'll develop the habit of moving. Your pinball days are over. 

When you serve, whether in a match or when practicing, you should always first visualize the serve in your mind. You should see the serve's entire path and everything it does before serving - the contact off your racket, its speed and spin, the first bounce, its path over the net (low, any curving due to spin), the second bounce on the far side, and where it bounces after that. You should also feel the contact in your head, whether a grazing contact to create great spin or sinking more into the sponge for more speed.

Then serve, and observe if the reality matches what was in your mind. This gives feedback so that you can learn to control and develop the serve. Most players who have trouble controlling their serve (i.e. most) do not use this feedback, and so it's no surprise they can't control their serve. This also allows you to improve the serve by visualizing as it should be, and striving to match that.

When practicing, perhaps play a game with yourself where you visualize different serves, and see if you can match what's in your head. Practice until you can do this with all of your serves, including both spin serves and fast, deep serves. Perhaps even put targets on the table and visualize hitting them, and then do so. Eventually you'll have the feel of any serve you can visualize in your head, and to repeat that serve you just have to match the feel of the serve that you've learned. 

If you watch the top players you'll notice that most have only one or two basic serving motions that they use over and over. The majority just do forehand pendulum and reverse pendulum serves. This allows them to do every possible type of spin, including sidespin both ways. Most have a few "trick" serves they'll pull out sometimes, but the large majority of their serves are almost relentlessly the same few motions, though the spins vary quite a bit.

But they are world-class players playing other world-class players, who are not particularly vulnerable to trick serves or varying service motions. That's not true of the large majority of players. Against them, you should also have only one or two basic serving motions that you use most of the time. But you will have a huge advantage if you can regularly pull out other serving motions, as long as you can do them effectively.

After a game or so, most players adjust to an opponent's serving motion. But what if you are able to pull out other ones, and keep them guessing? For example, after a few forehand pendulum (or regular or reverse), throw in a backhand serve, or tomahawk serve, or any of a zillion other possibilities. They key is not just throw out these serves as just "trick" serves, but actually learn to do them well. Otherwise they are one-serve wonders, which have value if used perhaps one time, but not much beyond that. Instead, develop these other serves so you can pull them perhaps out a few times each game, and perhaps get a few "easy" points.

It takes a lot of practice to develop multiple serving motions, and just as much time getting comfortable using them in games (where you not only can do the serves, but get used to the various returns). It's a lot of work for a seemingly small return. But is it really a small return? Players spend years improving parts of their games only marginally. Perfect a new serving motion, and you may find opponents struggling against your serves later and later into games, including those all-important points near the end of a close game. 

It's always been strange to me how a problem faced by so many players is so rarely dealt with. And that is dealing with distractions. Over the years, how many matches have you lost because something distracted you and you lost your focus? What did you do to solve the problem?

Most likely you just tried to keep your focus in practice matches. That's like developing your loop only by looping in practice matches. It's part of the solution, but you'll develop it a lot faster by doing drills that allow you to focus on that one aspect of your game. Similarly, if you want to work on your focus, find drills that allow you to focus on that one aspect.

I'm going to give one example of how to work on this. When Tiger Woods was developing and practicing his strokes as an up-and-coming junior, his dad would sometimes jingle keys or do other distractions. Tiger learned to focus completely on what he was doing, and the result was nothing could cause him to lose his focus.

You can do the same. It may seem silly, but why not sometimes practice or play practice matches where someone does exactly that - jingles keys, plays distracting music or news, or just talks to you. Your job isn't to just ignore him; that's not the point. Your job is to tune him out so you don't really notice him, so that your entire focus is on what you are doing, like an absent-minded professor who doesn't notice his house is on fire. You need to be so focused and inside yourself that you truly don't pay attention to outside distractions. When you can do that, you'll never have a problem with distractions again. 

If you think about it, this is somewhat obvious - and yet most don't really think about it when fixing a technique, leading to great difficulties in making these changes. They'll try to fix one part of the technique, but unless they fix both parts at the same time, the technique won't work properly.

Imagine a player with perfect technique. Now imagine changing something so his technique is no longer perfect. He'll likely have to compensate somewhere else in his technique for this change or his shots will no longer hit the table. Similarly, a player with one poor part of his technique will almost always have at least one other poor part to compensate.

For example, let's suppose a player's backswing is too short on a loop. To compensate, the player will likely swing more violently to gain the racket speed, leading to a poorly-controlled jerky stroke. To fix this he has to both extend the backswing while slowing down the acceleration to a smoothly controlled swing.  

Or suppose a player hits or loops forehands with his right foot (for righties) too far back. To compensate he'll likely have less waist rotation (since otherwise he'll literally be facing backwards during the backswing, leading to an awkward and less-powerful stroke. To fix the problem he has to both bring the right foot a bit more forward while increasing the waist rotation.

Or suppose the opposite, that the player hits or loops forehand with his right foot slightly forward (i.e. a backhand position) or even with the left foot (for a player who doesn't have a supple waist and/or spend many hours each week training like top players who correctly do this). The player will likely find it difficult to rotate the body properly in the time needed in a rally, and so will tend to stroke with only the upper body and arm, again leading to an awkward and less-powerful stroke. To fix the problem he has to both adjust the foot position and increase the body rotation. (Note how both improper foot positions lead to difficulties with using the lower body and proper body rotation.)

The same is true of a player who plays with his two feet parallel to each other, which makes it harder to rotate the lower body, and so leads again to an awkward and less-powerful stroke. Again, both the foot position and the body rotation need to be adjusted. Telling him to do one without the other won't help.

Or imagine a player who stands too far off the table when forehand looping against backspin, a common problem. He'll have to reach forward too much in his swing, and so to keep his balance will have to pull back with his left side, and so fall backwards slightly, leading to a loss of power and balance. Telling the player not to fall backward won't help unless you also tell him to stand closer to the table. His contact point with the ball may be the same, but it won't be so far in front of him.

Another common problem is the player who lifts his elbow during his forehand drive, leading to the racket angle changing from too open to closed during the stroke, making it hard to control what the angle is right at contact. If you tell him to stop lifting the elbow without also focusing on starting the forward swing with the racket in the proper angle he'll have great difficulties.

Sometimes there are more than two roots to the problem. If a player stands up too straight, this forces all sorts of adjustments to compensate. When the player tries to stay lower with a wider stance and knees more bent (as he should), he'll have to change a number of aspects (for the better) to his stroking technique. But if he doesn't make these changes, his new stance and stroking techniques will be awkward.

There are many more examples like this. What technique problems do you have in your game, and is the root of the problem singular or plural?