Many players have jerky strokes that make them look like a marionette on a vibrating bed. Jerky strokes mean the player is mostly using one muscle for the stroke, often at near full-power, which is nearly impossible to control. It's better to use more muscles smoothly, i.e. at less than full power, for control as well as more power since you are both using more muscles and are smoothly accelerating into the ball rather than trying to jerk your stroke at full power at the last second.
Jerkyitis is a horrible disease, according to top-secret papers from the Center for Disease Control. It cannot be cured. Like an accent a person has when learning a language at an older age, there will always be a bit of stiffness in your shots. But it can be controlled, just as that person with an accent can still develop conversational skills. How do you learn to control your jerkiness? By not rushing your shots, by relaxing your arm, and not trying to consciously guide your shots during a rally.
To avoid rushing your shots, you may need to take a quarter- or half-step back to give yourself more time to take a more relaxed shot. Players who suffer from jerkyitis often are jammed at the table, taking every ball right off the bounce, and doing so only grudgingly since they look like they to want to take the ball before the bounce. This is actually okay for a mostly blocking style, but for most shots, it's a symptom of jerkyitis; left untreated, your game may die a slow, malingering death.
Relaxing the arm is easy - you just need to do so. Before the rally, relax your arm at your side, then gently and smoothly raise it into your ready position. Okay, it sounds easy, and it should be easy, but the key is to keep it relaxed once the rally begins. There's a natural tendency to tighten the muscles as the ball is coming toward you as you try to guide the shot. Instead, just let the shot happen on its own - rely on your many years (months? weeks? days?) of training rather than trying to consciously trying to control each shot - which is the third thing you need to avoid to get rid of those jerky shots.
One way to practice smoothness is to do random drills. This means your partner or coach hits balls randomly to your side of the table, and you just react to each shot, using the principles outlined above. (You can do this either live or with multiball.) At first, have your partner or coach hit the ball randomly either to your forehand or backhand; when you can react to those shots easily and smoothly, then increase the complexity and have the shots go truly random, i.e. to the middle and wide corners.
We'll never wipe out jerkyitis, but if we all work together, we can wipe out much of its effects, and turn your game into an (almost) smooth-stroking table tennis machine. (In the interest of transparency, I am a lifelong sufferer of jerkyitis, but it hasn't stopped me from reaching a rather high level as a player and coach.)